I played killzone 2 before the controls were patched and couldn't get into it.
The mw2 single player is fast paced, and a little confusing at first. I didn't read anything about the plot before the game released, and have really enjoyed replaying levels to figure out the story. Combine that with increasing difficulty, and I've actually spent more time playing mw2 sp than resistance 2 even though the campaign is shorter. Not to say R2 isn't a good game, because I definitely enjoyed it. The graphics look good, it's reasonably challenging, the bosses are fun, and the multiplayer good. My only complaint is that the game was a little more predictable, and the multiplayer starts to feel repetitive more quickly.
I would recommend renting KZ2 and R2, and buy used if you do like 'em. I can give the go ahead for mw2, though.