Why not both?
You can pick up Vice City used for $5 at EB Games
I'm a huge fan of GTA (I own 1, London, 2, III, Vice City, San Andreas, Liberty City Stories, Vice City Stories, and now IV [the complete Playstation GTA collection]) so I love them all, but go with IV if you have a 360 or PS3, since GTA is known for horrible graphics before IV. Now GTA looks absolutely wonderful in HD goodness.