4th Edition
The main difference I see is that 4e is a little more explicit than previous editions as far as what's expected of you, both as a player and as a DM.
On the player side, if you're in -- for example -- the Defender role, it spells out exactly what that role means, and it's right up front how your class powers fit into that role. In 3.5 it was always a little bit mysterious, like the authors didn't want to just come right out and say, "Your job is to suck up attacks and occupy a small number of strong enemies." Some people think that makes the game MMORPG-ish, but I don't see it. It's not saying you can't do anything else; but having that information right up front means if I decide to give my fighter a more striker-ish build, I know how that's going to change the party mechanics. Or, for that matter, if we don't HAVE a defender, I can predict how that might affect the group.
On the DM side, 4e is much more up front about desired end results. In 3.5, you want to build a monster, you go through all this rigamarole with hit dice and type and attack bonuses and stats, and if you put in all the right ingredients, and if you turn the crank the right way, you'll probably get something that's kind of close to what you wanted.
In 4e it's just kind of "Level 12? AC 24, attack bonus 15, dadada, feel free to adjust those by a few points." It just tells you what the answer should be without worrying about how you get there.
Encounter design is kind of the same way -- you get your XP budget and a couple easy limit values (in terms of encounter level -- and thus XP budget -- relative to party level, and in terms of individual monster level relative to party level), and you build your encounter. There's no CR calculations to play with.
Similarly, the monsters themselves are clearly labeled as what they are -- artillery, soldiers, brutes, leaders -- rather than showing you their powers and waiting for you to work out what they're supposed to do in a group or what their tactics should be.
My one complaint is that there's a lack of flavor text on both monsters and PC abilities. I'd like it if monsters had a few paragraphs of fluff, but I understand the reasons they didn't include it.
Pathfinder
I only looked over Pathfinder for one afternoon, but here's my thoughts:
Pathfinder feels like 3.5 Only More So. In the effort to balance the original 3.5 core classes with later material, they've bogged the game down in ridiculous quantities of little fiddly bits.
For example, the bard: Just about every level, the bard learns a new type of bard song. Some of them are upgrades of previous songs, but most of them are just adding new options. Each race gets a shopping list of +1s and +2s, and many classes get the same.
What it adds up to is enhancing the worst part of 3.5 (in my opinion, at least) -- the huge amount of bookkeeping you need to do, both as a player and as a DM. You have a dozen little adders and bonuses to keep track of and forget to apply, many of them conditional; and you have a huge number of options for what to do with your action each round. You've only got five or six rounds in a combat, so it seems to me that 15 bardsong options is kind of overkill.
Even the old "easy" classes, like the barbarian, have been complexified. How does this strike you? Every day, your barbarian gets a certain number of Rage Points, and he can spend one to rage for one round, and then he can spend a few extra points to get additional special powers while raging... So the big dumb guy class now requires me to track points? If I wanted that I'd be playing a psion.
All the classes strike me that way -- Nothing is elegant or simple, rewarding cleverness. Everything is keeping track of uses and points and bonuses that all change on a round by round basis, and everyone has a list of class powers that puts the wizard to shame.
Don't get me wrong. I don't mind power lists. I like 4e's power system; it's elegant where Pathfinder is clunky. Probably this is more than partly due to 4e using a spell-description type listing rather than infinite reams of dense bullet-pointed text lists.
I guess I also have a sense, perhaps not well justified, that the reams of Pathfinder class powers don't hang together very well. There doesn't seem to be much synergy between various class abilities to allow the player to perform a particular job (unless that job is "deal damage"). I could be wrong on this one; like I say, it's just a feeling.
Anyway. If you liked 3.5, then Pathfinder is probably a solid system. It won't blow your socks off; it's nothing especially revolutionary. If you had some serious problems with 3.5, well... again, Pathfinder is nothing revolutionary.
Which one would be best for you, however, depends on your own style of gaming...not just playing, but DMing as well. /snip
