SFIV is easier control-wise, it's more lenient (not as exact) on directional moves. I believe Capcom adjusted the controls for the 360 controller for this game.
That's what pissed me off about playing SF II HD Remix, especially with the 360 controller is that you had to be ridiculously precise and quick most of the time. And the d-pd and smaller jotstick were ill-equipped to handle it right. Considering I could play SF Anniversary Collection and Alpha Collection like a pro with the PS2 controller, it really shows how sad the 360 controller is at handling fighters.
So far, I'm getting a good kick out of SF IV though, but Seth on medium (normal) difficulty is totally cheap and overly difficult. My advice: master blocking and timing your moves, because he will knock you out of the air if you just charge in reckless.
I really having fun with the online matches the most, and the lag is pleasantly minimal.