Windows 7: Basic-ish system, based on Windows Vista. Aero transparency. The only version in the comparison which actually has a non-Aero mode. No support for Modern UI apps. Pretty stable. A bit old but still works, though I don't recommend it on newer PCs. Highly praised by users.
Windows 8.1: Its predecessor, Windows 8, has introduced Modern UI apps. 8.1 fixes on a few problems of 8, interface-related. Modern UI apps require Aero compositing and that will work even in software rendering now (of course, it won't be fast). There is no way to avoid compositing in the 8-family. 8.1 has introduced the automatic downloader of components such as NTVDM or .NET Framework 3.5, where a prompt appears when they are needed. Slightly better than 8 but not quite liked by its users (it's great on tablets though)