Honestly, for me, if it's available in a number of formats, I'll take whichever one is least expensive
I've found that there can be some significant price differences, and with Fictionwise's microrebates program, you can apply part of what you spend on some books to buying another, so that gives them kind of an edge. The PDF stuff is a bit oddly formatted at times, especially since I need to enlargen the type to read it, but if it's gonna save me $3 or $4 bucks, I can handle it not being all pretty-like