Well, seeing as I have not really played GTA3, I would say True Crime. You have quite a bit of freedom, as you can go along with the storyline, or you can just f around. Personally, I like the skill system, how you upgrade by the fact you have to earn it with practise and badges.
Another thing I like is frisking people. Some people have funny reactions.
Nick Kang's one liners can be funny, and from what I've seen in GTA3 the guy doesn't talk as much, so *Shrug*. But then, True Crime is shorter than GTA3, as you can beat it in about half a day, where as I've heard it takes longer to beat GTA3.
Well, give us/me your input here folks.