I actually have no experience with slim external drives, but i have used a sony writer with enclosure at the office, i actually preferred the enclosure type because it a lot cheaper before (externals used to be more than 5k), it was easier for me to ask for office approval.
Disadvantage of the enclosed one for me was it was too bulky. go for slimmer external if you prefer a more portable one, besides the slim type is more pleasing to look at. But I've read somewhere there were burn problems with a usb powered one, maybe they corrected this already. at may warranty naman
Let's wait for concrete feedback from someone with actual experience with slim types.