Guess it depends...IF you are going to be storing large files, or booting off of it, go for whatever has the highest RPM.
Probably 3.5" @ 7200 RPM for externals... 2.5" @ 5400 RPM would be good...but I bet most of them are the generic 4400RPM lappy drives. This is all assuming your using USB2 or FireWire. If you're using USB1, I doubt RPM will matter to you at that point, esp if it's just for storage.
I was using a 4400RPM 18GB drive in a 2.5" enclosure...I trie it under FireWire, USB2, and USB1. For regular file access it was all good. when copying large files, FireWire was the best, and would only be better as the RPM increases.
Just my $0.025438 worth.