I agree with Aerozeppelin about not being able to see half the time. Between the sunglare and the shadows you can't see much, then when you're in your car you can see anything directly in front of you because of the way the camera is positioned behind the car, this leads to ALOT of wrecks for me, If the camera was just rotated up about 10 degrees so the car doesn't block everything closer than 20 foot in front of you. First person mode Definately helps but adds it's own restrictions

I disagree about vice City looking better though, It was probably the xbox version that looked better.
San Andeas adds ALOT to the gameplay department, and the story is good even though I'm not the biggest fan of the setting. The Economy in much better in SA also. You aren't rollin in the dough because there is always a ton of stuff you need/want to buy. Also the game lets you personalize your character.
I think the games are equally as good considering Vice City is a few years old. SA took existing gameplay and made it better, much like Vice City did.
edit: Aerozeppelin I've also noticed the targetting system being a little screwy, I've looked directly at a cop 5ft in front of my and I target and it targets a pedestrian 100 yards away behind me. It's got me killed once ot twice.