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Gameplay: The Gameplay doesn't have such a dramatic change between the two games. Grand Theft Auto 3 was harder, and it was a little better since it was the original, it deserved more credit as well. The cities in GTA 3 felt better to roam around than vice City too, IMO. Where as in GTA Vice City, we had more vehicles, weapons, and gangs. Although the city wasn't quite as fun to travel around, the new vehicles, weapons, and the gangs were much more fun to mess around with, which added countless hours to the gameplay.
Graphics: The graphics, compared to the PS2 versions were much better. Even though so much things can be going around in the background, the framerate persists, which is great. So the graphics, for both games, is a big bonus.
Sound: The sound of everything in both games adds as well. Everything has its own unique little sound, and it's quite realistic. So there's another bonus.
Value: The Grand Theft Auto Series is really not known for its missions, but rather its open world. Most people I know never even beat the game, they either cheated there way through, or just mess around the game, or both. Most people go exploring looking for random things, getting into police chases, trying to get as many stars as possible, trying to live as long as possible, or go on a killing spree. Just messing around with different stuff adds countless numbers of hours. This is what really brought both of the games success. Adding some sort of multiplayer would've also made the game even more popular, and added tons of replay value, but you'd have to wait for GTA Vice City's sequal for that.
Reviewer's Tilt: Although the port didn't really bring up anything new, the games were revived. Now Xbox owners can enjoy the greatness of both games. They deserve a lot of credit for opening up doors to such open world adventures. Although there were other open world games, this was one of the best out there, and both games will probably hold that title for a while. Not even its sequal IMO, can take that away from these two games. Awesome games, if you don't have it, GET IT!