I think Resistance 2 will be much better from a technical, game design perspective and story telling perspective. But I expect Microsoft to generate much hype and the US press to eat it up like they did with the original Gears of War.
I would think Gears 2 would initially outsell Resistance 2 for this year, but R2 will probably outsell Gears 2 lifetime sales. My hope is on Killzone 2 and Resistance 2 to pull many new hardcore FPS fans towards the PS3. Sadly FPS games don't do well for Japan, so that PS3 market advantage will not be significant.