I liked Resistance 1 a lot, but IMO Resistance 2 is way better in most ways.
I still recommend friends to play R1 first for its fun gameplay and interesting story telling, this to prepare for Resistance 2.
However since playing Killzone 2 this upped my desires for an FPS quite a lot. I am currently trying to finish COD: WAW, but graphically / audio wise and gameplay style, albeit still good overall is well below what was done with Killzone 2 for me. I think maybe I should have played that game before playing Killzone 2, this to be more forgiving (maybe the same will be true when returning to Resistance 2).
Can't wait for Resistance 3 and Killzone 3!