Just the old one. On PC it looks lovely maxed. You can probably pick it up dirt cheap. The definitive edition on PS4 just ups the original last get console versions to match the original pc version and includes the dlc. I'd pick up the original for pc if you have a decent machine.
I dunno. I have it on PS3 and haven't finished it. Once you play one gta you've played them all really, imo. I found the likes of sleeping dogs to be better, much tighter controls, some shenmue style fighting.I know I'm in the minority.