Ok so I have been to all the Disney and universal parks (yes those are the only ones I call major) in the USA more than once and here's the low down. Disclaimer: I moved to Orlando this year and have been to the Orlando parks cumulatively like no joke 300+ visits in my life. But I've been to the Cali parks more than once now and as an adult so here we go.
1: you can make a vacation to the region either group are in (Orlando or LA), but the LA parks should only be your vacation destination as part of a full blown LA trip. There's only three total parks versus 6 in Orlando and they are smaller at that so you can cover both Disney LA parks in one day. But LA is a cool destination so just know it depends on what kind of vacation you are in the mood for. For an all out impressive theme park vacation, you're going to Orlando. For the LA parks, you need to plan a varied trip that does more than the parks in that region.
2: When it comes to the kings, all the Disney and Universal parks are worth a visit in your lifetime as a US resident because all have something unique. Even the duplicated parks - Universal Orlando has the fantastic Diagon Alley and city faux backlot where you can take the Hogwarts Express over to the other park, but Universal Hollywood is in the Hollywood hills and is literally connected to Universal Studios backlot where they give you an excellent tour of real famous sets and around the sound stages of active shows (Universal Orlando used to be a working studio too but no longer is). The Magic Kingdom doesn't look like a tiny shitty movie set and has the prettiest castle and a sprawling size, but Disneyland has been
3: Okay, yes, Disneyland is rad in some ways as it has a cool Louisiana area and a better Star Wars Space Mountain and some other different stuff.
4: Here's a list of the Orlando parks and how they are-
Magic Kingdom: it's pretty, quite big, and polished above all the other parks. There are lots of famous Disney properties, sweet music, and nothing feels like it is there simply because it is too pricey to renovate. It is also, however, the most kid oriented park. The roller coasters are cool themes and architecture but not extreme - they exist to bring you into an atmosphere of PG-13 and under fun. The edgiest this park gets is Pirates of the Caribbean and Haunted Mansion, which are both safe and just spooky. Features several very well contained zones that go above movie set cheap looking and stuff like a little frontier island you have to ride a raft to get to. Overall, this park feels fairly plush, friendly, and sweet. It's wholesome and you paid a lot for it.
Animal Kingdom: this one suffers from what I call 3rd world simulator. There's a dumb fixation at many theme parks (especially less established ones than Disney branded ones) to essentially build a place that looks like an impoverished African nation. Much of Animal Kingdom is this. You walk through supposed Safari villages that glorify some less exotic zoo exhibits while Disney says please come back when we get that Avatar zone finished. That said, there is a great legit Safari type experience on a massive habitat, an impressive looking humongous fake tree, and a few other genuinely good rides (Dinosaur is surprisingly scary and intense, and Everest is a very fun coaster). This one is a little more fun for adults looking for a different kind of big park as it has the money of Disney but stays away from the now common fairy tales and action movie themes better when it comes to atmosphere - but that means it does remind me of zoos and Busch Gardens kind of tier parks that struggle. It's the only Disney park that I would say you can skip if you struggle with the cost. Overall this one is pretty family friendly, but at the risk of exhausting with its atmosphere being that of no luxuries.
EPCOT: the best park for a couple who likes to romanticize traveling or an adventurous geek - but don't think it's adult focus makes it a substitute for seeing Magic Kingdom. It starts with a sci-fi heavy premise of futuristic ideals and rides to engage that excitement, then it sets you off to its best feature which is the lake that features a number of beautiful snapshot pieces of notable countries. You can spot landmarks from Japan, France, Mexico, Germany, England, and more across the big lake before you even start the walk. Each zone has something to see, from artifacts and quality brief films and food, to rides like Frozen in Norway. Now this park has a few killer rides, but most of them are the science and futurism fare, not in the countries where you get the cool cultural exhibitions and vignettes. But a nice German beer and a walk is likely to please you after you finish the rides at Epcot. It's a beautiful festive park.
Hollywood Studios: this is sort of their cool park these days. Soon to be home of Star Wars land, it has some great rides like Hollywood Hotel Tower of Terror (not that Guardians of Trash Chute that Cali is getting), Rock'n Rollercoaster with Aerosmith, and the Star Wars lite zone that will hold you over before the big zone opens. It has great old Hollywood looking streets and a fun bit of movie magic type homages, like a famous Indiana Jones show. It's cool and has the most nostalgia built into it as a love letter to Hollywood and your favorite big movies that Disney can touch. This one is the best mix between adult and kid thrills to me. The adults see the old world charm and the kids see their Star Wars and Pixar favs everywhere.
Universal Studios Orlando: first I will say this park is on the up and up, with a new Jimmy Fallon experience and a Fast & Furious ride on the way (though that one is likely another weird copypasta from California that I'll touch on). So I actually have an annual pass to the Universal parks and it's tough to explain why, but the answer is probably the Wizarding World because it is that good. Studios is A decent park but plagued with a mix of licenses that you know Universal doesn't own, some stuff that is obviously from back when they had no money yet, and some rides that straight up have no licensing and are just generic theming (Monster Cafe for no apparent reason that has no resemblance to the film franchises they are pushing, Rip Ride Rocket is a music coaster that seems like a weird inspired-by of Disney's Rock N Rollercoaster except it is outdoors). The Simpsons area is simultaneously the most interesting food and worst ride. It actually is the worst ride I can recall that still exists in all the parks I'm discussing. There's a greater problem here at fault but we will come to that when I discuss the other Universal park. The park has a lovely Hollywood street and an even lovelier New York backlot that pays host to the Brendan Frasier heavy Mummy ride for whatever reason. Harry Potter brought Diagon Alley as the newest area and it's fucking great. This is where universal is a champ. They have put the most beautiful quality detail and atmosphere into any theme park areas ever with how well they've handled the Harry Potter stuff. The buildings and merchandise and lightning and music are brilliant. Gringotts is gorgeous and it really looks like a surrealist London. This is what you came for tbh, and now let's literally take a train ride to the other attached and more popular little sister...
Islands of Adventure: A rad park that seems to be the park based on Blockbusters (and some shit that no one recognizes because it is again remnants from before these parks had money). There's two areas with their days numbered which are the Lost Continent (generic mythology place with no licensed properties, but hey there Nintendo) and Sunday comics whatever (very few famous comics involved, just like Betty Boop and Rock & Bullwinkle). Another that's in a weird state is Marvel, which can obviously not change much and is stuck with only the comic book art, with no MCU stuff. So it is plastered with some very 80s looking art and horrible costumes for their heroes and the redone Hulk ride feels like a good try that was aiming to leave behind the comics but had to do a generic animated TV series approach. So that's it for the heavy downer stuff because the rest is quite charming. Harry Potter's first foray is Hogsmeade/Hogwarts which has a beautiful castle and snow covered village that can be reached by a cinematic literal train ride on the Hogwarts Express. The ride is great and the castle inside has all the movie atmosphere. Butter Beer is the one thing I'm guaranteed to buy every time. Beyond that, Spider-man is a fun ride, Kong is cool... check online for lists of all the specific rides but let me come to my beef with Universal. They cheap the fuck out on rides in the form of simulators. The vast majority of their headliner rides are simulators. In Studios: Transformers, Shrek, Minions, Harry Potter, Simpsons, and soon Jimmy Fallon, Fast & Furious (im probably forgetting something). In Islands: Spider Man, Harry Potter, Kong. You put on 3D glasses and experience minimal coaster and moving set pieces. They're the best simulators, but they are simulators and several even have a formula they share that you will catch on to.