Argh, I had a reply carefully typed up, but it succumbed to some stupid bug that hung the browser window. I'll go from what I remember.
I pushed too many hours into Disgaea before playing La Pucelle, and as a result I was unable to finish La Pucelle. As Soul4ger said, if you have the time, play them in order.
Between Phantom Brave and Disgaea, it really depends on what you're looking for in an SRPG. Phantom Brave has many more gameplay elements than Disgaea, and the AI has improved a bit so that I feel a bit pressured. I'm also less likely to have uber characters in PB because of the limited time that a unit can stay on the battle map, so I have to train a backup set of units as well as employ extensive use of my supply lines (read: healers). As a result, my units are more balanced than in Disgaea, where I have one unit at level 2500 (transmigrated ten times) and the next best unit at 400 (three transmigrations). The Tactics Ogre style turn system in Phantom Brave forces me to actually play with some sort of strategy (although in the later levels it's a case of how fast I can kill enemies), while Disgaea's strategy is to maximize the amount of damage dealt per turn (to be fair, it's because the combat system is easily abused, not because its turn system has the same style as Fire Emblem/Advance Wars).
On the other side, Disgaea has a more charming story, and its battles don't leave me with the feeling that combat is detached from the narrative. In Phantom Brave, I get this strange feeling whenever I get transferred between combat and cinematics -- it's as if I'm play separate games that share the same universe. This is also amplified by the fact that the units I create in Phantom Brave not only are too generic (they rely too much on the weapon they wield rather than their base abilities), but also don't participate in the story at all. At least with Disgaea, I feel that my units actually play a part in the story.
Personally, I enjoyed Disgaea more than Phantom Brave. The level of micromanagement is greater in my opinion, and I *like* creating a small group of powerful characters. Being an Elitist Linguistic Prick, I also preferred the mix of Japanese and English voices in battle rather than Phantom Brave's purely English battle cast. (I think I would have gone insane if they replaced the archer's and ronin's voice with potentially inferior battle cries. It's bad enough that I have to deal with Jennifer's pronounciation of "coup de grace" [the "p" is supposed to be silent]) Mind you, I wonder why neither of them offered the pure Japanese voices in battle like La Pucelle.
Anyway, that's the end of my rant.
Edit: Oh yeah, you really don't need to power level to beat either game. You should do it if you ever plan to get the best items/secret characters in either game or get all of the endings in Disgaea.
