I think USB would probably be the most convenient for me.
I mainly want to use it to store my music, to keep it off of my main drive and so that I can plug it into other computers if I need to (I'm going to Germany this summer and I think I'll probably buy a netbook to take with me (and just to have, that's the excuse

) and it would be convenient to have everything on an external drive).
So size isn't a huge deal since it generally won't be going places, however portability is a consideration (of course since that's one of the major upsides to an external drive in the first place).