Well, in Jak 2 you can just run around Haven City with the guards chasing you, without having to worry about KG robots or Metal Heads, which adds a lot to the gameplay.
I'd get Jak 2, if I were you. It's a lot of fun that you'll most likely enjoy.
I have Jak 3 and it was pretty good. I wanna know if Jak 2 is any better, or the same thing.
Jak II was a superior and funner game IMO. The story line was better, the characters had more life (looking at you Keira, Jak 3 was no slouch in this department however), and the game featured more platforming and was longer. Jak II seemed more faithful to the original gameplay-wise as well.