It really doesn't differ. Retail can be a little more convenient, but hardly worth the additional money, unless you actually plan to RTFM. Even changing MBs is no big deal. You'll hear horror stories, but I think they're urban myths because I've called MS several times over for that very reason, and I've never had a problem.
I guess my main concern is about the availability of upgrades. Are there problems in getting upgrades? I was concerned with what Microsoft calls "support".