Graw1 was more challenging than GRAW2. The atmosphere in Graw2 is better. Graphics in Graw2 are better. Get both! But if you only have $$ for one game, then get Graw1. Then, when you get get bored, get Graw2. Graw2 was dumbed down for the masses.
Yeah get Graw 2, has a bigger online community and that is what you should buy these games for, the online. The single player is ok but nothing really special, once it's beat it's not a game that makes you want to go back and play it again. Unlike the online co-op or something.
Plus if you go for the achievement GRAW 1 has 3 unattainable ones, so skip it.