There is nothing much to look at except you want them to run on external power or not.. If you want the ones powered by only USB get the 2.5" ones or else if it's going to stay at one place get the 3.5" ones.. Prices are different everywhere so it;s best if you went shopping asked at a few places and then choose the best one, Buffalo, Seagate, Transcend, Verbatim, IOmega make good quality...