Depends on how many RPMs the fans have. Go for the one with a higher RPM.
Actually, it's often not about the RPM's. Many fans with lower RPM's have higher CFM's and higher static pressure. It's about the design. A few RPM's doesn't hurt though. Heh. And actually, I just tried a Noctua NF-P12 on my 212 EVO heatsink and my temps actually went UP, although it was noticeably quieter.