Of course, Resistance was the best launch title, and the best game on PS3 for a good 6 months, so it would sell well. Plus, it was bundled heavily. It sold well for AGES. Serious legs.
Resistance 2 looks to be a much better game, but it has a lot more competition, namely, as you said, Call of Duty. Although Resistance was competing with COD3 and destroyed it, so...
SO, basically, R2 can sell extremely well if COD5 sucks. Which it just might, considering Treyarch's history.
Also, a lot of people were put off by RFOM. But a lot of people are going to pick up Resistance 2 having not bought Resistance 1 (I had a choice between RFOM and Sigma, WHY DID I CHOOSE SIGMA *banghead*.
Also, please stop calling it RFOM2. It sounds ridiculous. It's just R2. Or Res 2.