Must say first that I have no experience on how "fat" SLED is, but I'm pretty sure that most of the resource-hogging is done by the desktop environment and X these days. And since SLED used (as far as I know) KDE by default and Ubuntu uses Gnome (well, Kubuntu uses KDE too), which both are quite big (pretty nice too, anyway), so I don't think they make much difference. If it's just about the performance, I don't think you'll see big difference. I myself would maybe prefer Ubuntu, but that wasn't asked
With those specs either of the two should run. More RAM is more fluent day-to-day work, but 256 should do unless you're a gamer.
