Well....
Resistance has the more innovative weapons. Hands down, they're just cool. However, Gears is like... 100x more fun to play. That's my opinion though, maybe you like the more traditional shooter? I love em too, but Gears is SOOOO much fun.
They're both great games, I just wouldn't call Resistance a system seller, I'm getting a 360 so I can play gears. So... yeah.