Personally I'd recommend jumping onto the Pathfinder bandwagon. their core system is a bit tighter(more internally consistent) than 3.x (though I'd prefer it be even tighter than it is), I haven't seen any of their changes as worse than 3.x.
I know the compatibility described above, because I do alot of the reverse. I play and run PFRPG, and I want to be able to use much of the 3.x content (as a DM AND as a player).
Pathfinder characters tend to be "BETTER" than 3.x characters, but not necessarily more powerful. They tend to have gained a pretty slight power boost, and a big boost in utility. You have more options, but not really more powerful options.
A few classes got their balance shifted up or down a tad. BAB is tied to HD now, so everyone gets a d6, d8, or d10 (Barbarian keeps the shiny d12).
I like the PFRPG System better than 3.x, and everyone I've played with who knows 3.x ends up liking PFRPG better after they try it (except when they say "I want to use X from 3.5, but PFRPG doesn't have it.")
Note: While compatible, I know alot of PFRPG DMs who won't allow classes from 3.5, worried that they will be over or under powered (this is mostly superstition). Personally I tend to allow but require converting (usually slight changes), then I add it to my ever-growing library of usable PFRPG options.
You can reverse engineer it back down to 3.5, but it's worth it to convert the 3.5 you want up to PFRPG instead. As explained above, alot of the stuff is usable as is, or the conversion is pretty easy, Classes being the only part that take much work.
Monsters in PFRPG are better balanced for their CR as well. There's some mathematical analysis at the back of the bestiary that makes it alot easier to make new monsters.
One thing I should mention:
LA no longer exists.
It wasn't a very good system to begin with, and Paizo dropped it entirely.
The slight power boost that races got puts them around the +1LA mark though, so +1 LA races can be used with little change.
For people who want the higher LA creatures for players, I'd recommend building a custom base race and then a player class that emulates the monster's abilities (I ditched LA a long time ago), but I know most people don't want to go to that much work to play the monsters they want to play. Here's the
best option I've seen that's already done for you. Designed for 3.5, but it works just fine in PFRPG as well.