So I just played around half an hour of the Resistance 2 beta. It's no comparison. Killzone 2 is vastly superior, though Resistance 2 should hold it's own for "run n' gun" fans. Resistance is extremely fast paced, like the first, but with the addition of squads, more random spawning (you spawn by your squad, and even then sometimes you can't find the squad), and other stuff it can be overwhelming at times. The initial character models for both humans and Chimera are rather generic, as well. I'm not sure how they'll look as you advance in rank, but at first the just look bland. They also give you no time to adjust your appearance before the round starts. In Killzone 2, upon entering a game, you can adjust your class, weapons, etc., and then start playing when you're ready. In Resistance 2, you could be in the middle of picking your weapons when suddenly you spawn on the battlefield (though that may have just been a fluke, since I hadn't played very long at that point).
At this point, I'd definitely say that KZ2 is the better overall game, though I may be somewhat biased in my preference for a bit slower paced and more tactical gameplay.
Really, the difference is caused primarily by Guerrilla's emphasis on realism. In KZ2, your character has real weight to him, the weapons feel like actual weapons, the camera wobbles as you run, etc., and the gameplay is designed around this. Resistance 2, however, falls more on the UT end of the spectrum, probably landing around Halo, except it doesn't play as well as Halo, imo. You run fast as hell, when you jump it's as if you just started to levitate for a few seconds (as in, your character's posture barely changes at all), you can fire while jumping, etc. It plays like an old school shooter, imo. The main plus is the insanely cool weapons.
I do have to give props to Resistance for it's community features. The buddy system is cool and the connections to MyResistance.net should be great, but Killzone 2 is no slouch in this regard either, and many of it's community features I'm sure have yet to be revealed.
Of course, this is just comparing the online. I enjoyed RFOM's single player overall, despite being underwhelmed by it's online, so R2 could really impress in that area.