What's the diff between Batman and Detective Comics and Superman and Action Comics?
Different writers, artists and stories.
Batman is written by Tom King, it's his solo book right now, he's just about to start its third arc, the first one being "I am Gotcham" and the second being "I am Suicide". It's primarily focused on Bruce himself, though it does have a pertty large cast depending on the arc. This last arc had a lot of focus on Catwoman.
Detective Comics is currently the Batman Family book so to speak. It follows him and Batwoman leading a team of less experienced vigilantes, it's my favorite current bat book, and it just started its third arc.
Superman is focused on Superman being a dad, and his adventures with his son Jonathan. It's an absolute joy to read,
Action Comics deals with a lot of stuff happening in Metropolis, but it's main focus is on cool battles, and Lex Luthor trying to take up the mantle of Superman, while Superman himself doesn't trust him.
All four books are great, though if I had to pick 2 it could be Detective and Superman.