IMHO both didn't push the SP campaign side as much as they might, and it seemed the bulk of the effort went into the MP. I'd say RFOM2 pushed more features while Gears 2 feels more polished.
Both are great fun and very good games though. RFOM2 is an 89% for me and Gears 2 a 90% (RFOM2 lost 1% for a few areas in SP where graphics were rough).
Try to be reasonable... its easier than you think...