I purchased and played through both Resistance 2 and Gears of War 2 last month, and although I agree that both games should not be compared directly against one another, they both involve plenty of shooting, and to me that puts them in the same category.
I had a blast with the single-player of Resistance 2, which I felt was much more intense and involved than the story in the first game. Being with your four enhanced squad-mates (I forgot the name of the group already) made the experience fuller, and ultimately with the awesome locales and the tough challenge, it was a memorable time. The competitive multiplayer was very bland and banal, and the cooperative play is addictive when playing with friends (or without), but it seems very much like a fast food-type gametype, where you get filled up as you expect, but ultimately it's not that stimulating. I might play some more, I may not, I'm not super-enthralled as is.
As for Gears of War 2, I had a blast playing through the whole campaign co-op with a friend. I had just finished Gears of War 1 before getting into GoW2, and although the game is very much the same, the cover and shooting systems are so riveting, I never got tired of playing both games. I will most assuredly replay both GoW's campaigns on harder difficulty modes in the near future. The competitive part of Gears of War 2 is very cool and fulfills that itch for carnage that crawls on me every once-in-a-while.
The best part in any of those games is undoubtedly Horde mode in GoW2. Essentially a survival-mode, you face up to 50 waves (rounds) of a mix of enemies in any multiplayer map of your choosing with up to 5 players total. Playing with friends seems to stop time, and the heart-pumping action really satisfies. It's a nice fact that you're able to restart the Horde game on any wave you've previously beat, and the need to setup some shields as a blockade for those huge Boomer or Mauler bastards in precise areas of the map gives the game a little tactical flair that is quite unique. Think Resident Evil 4's "Time attack" mode, only greatly expanded, and with 3-4 buddies. It's a bloody riot.
With all that, I've really had a great those of shooters in the last few months. RPGs are what I bleed still, and with Valkyria Chronicles, my heart can't stop pounding! I'll blog up about that Japanese gem shortly...after playing some more!