In my opinion, Pathfinder improved upon 3.5 a great deal while keeping the things I liked about 3.5. The balance of powers is still similar (as it has been for a long time in D&D), but I think it is better balanced in Pathfinder than 3.5.
If you're talking high level, then the most powerful classes are still the straight casters--cleric and wizard (and some of the other optional straight caster classes are pretty powerful too). If you're talking earlier levels of the game, then your fighter types are still more powerful.
Personally, I don't have a problem with this as players get to choose whether they want power right away, knowing that down the road they may be outshined by the casters, or if they want power down the road, knowing that in the beginning they'll be outshined by the fighter types.
That said, I don't think the spread is as wide as it was before. Casters get some help at the early levels so they're not completely useless. E.g. wizards have a low-powered magical attack that they can use in addition to spells. The fighter types get some love at higher levels as well.
One thing that is different in Pathfinder is the game gives incentives to play a single class. You can still multiclass and pick up prestige classes, but there are good reasons to stay with one class. You get bonuses every time you level your favored class (which you get to pick when making the character). The classes are all more attractive as well. Pretty much all of them have been fleshed out with additional special abilities so every time you level you feel like you get something. Dead levels have for the most part been eliminated.
If you're going to play a D20 system, I'd definitely go with Pathfinder. As you said in your post, there are many ways you as the GM can keep things balanced between the players so everyone's having a good time.