The Steam version is mostly for convenience, but it also lets you install it on any machine where you use Steam, so it's less restrictive than the normal Standard (single-machine) license. If you're just using DisplayFusion on home computers, then the Personal Home license is usually the best choice.

As for Windows 8/8.1, there are some improvements to Windows Explorer and whatnot, but as far as DisplayFusion goes, it will operate the same on Windows 8/8.1 as it does on Windows 7

I run Windows 8.1 here, as I need to be on the bleeding edge to make sure our stuff works, but maybe some other folks here can weigh in on whether they think the upgrade is worth it.

Please let me know if you have any other questions at all!