I was in the fortunate position of being able to play both these games back-to-back last night.
As far as gameplay goes it really depends on your preference in games - Resitance is faster paced, Gears has a more strategic approach using cover, etc. As scotty mentioned above Gears is far more refined - the graphics are perhaps a little lower in polygons than Resistance, but the textures and lighting/shadows are better making it look more real - some or all of that could be due to the TV/Connection tho - the XBox was running on a better TV using VGA and the PS3 is using HDMI but the TV is not as good.
Bottom line is they are both excellent games, but when it comes down to it I'm a PS3 owner and if I had the dosh I would buy a 360 for Gears, but if I had only a 360 I probably wouldn't consider getting a PS3 on the strength of Resistance.
Just my view,
Chris