LP1 has the better singleplayer campaign, but as both a LP fan and a co-op fan, I absolutely love LP2 (co-op buddy and me love LP1's campaign) and prefer it to the first. In some areas, it is weaker than the first (the sandbox-y feeling of some of the more open areas, the sense of urgency with the ever-depleting T-Eng), but the co-op makes it such an awesome experience that it's still fighting for top spot for my GOTY (vs Bayonetta or Dead Rising 2 or Mount & Blade Warband, I don't know).
If you are *both* a LP1 fan AND a co-op fan, this is a top top top recommendation. If you are only one or the other, potentially approach with caution. Also, the patch fixed some of the major 'issues' people had with the game.
I'll doubledip during a Steam sale so we can get 4p sessions going with 2 other PC co-op buds.