If you want portability, go with a 2.5" enclosure. If you want capacities beyond 160GB(200-250GB are nearing release though), go with a 3.5" enclosure. Most of them perform about the same as far as speeds go. They're mostly limited by USB/Firewire bandwidths, which are something liek 30-40MB/s. Go for an enclosure with a reputable bridge chip, such as Cypress, NEC, Oxford Semi, Genesys.