I think there's a lot of voodoo and superstition involved with UPS's driving computers. My suggestion would be that if you're buying new, you might as well go for pure sine, simply because it's one less thing to think about. If you already have a stepped-sine UPS, just keep it, or give it a try and see what happens. (Any decent modern power supply should not be damaged by stepped-sine; it might whine, or at worst, fail to start up.)