Tough to formulate a valid opinion without having both games running in front of me.
R2 is clearly a huge improvement over Resistance, so in that respect, it should be more impressive to those who liked the original.
Gears 2 on the other hand, while noticeably improved (it looks much cleaner for starters, as opposed to the "wet muddy" textures that were wrapped around just about everything in the original), is not as big of a jump visually, simply because the original had already set the bar extremely high. It still has some of the best visuals on the 360, two years after it was released.
All the shots circulating around would seem to suggest that Gears 2 will set the bar again for the 360 with some of the best visuals yet seen on the platform.
I'm buying both either way, so the real test will ultimately be which game will I end up playing more due to being a better overall experience in terms of addictive game play, interesting weapons, interesting story/characters and overall fun factor.