I personally have windows 8.1 on bootcamp on two computers, and I like it. It is really almost 100% personal preference. If you are currently a windows 7 user, you will probably want to stay with that. If you don't use windows very much, check out 8.1. I think it has a simpler UI, and is more similar to OSX. Backwards compatibility is typically pretty good, but they got rid of windows xp compatibility mode on windows 8 (you can still vm windows xp for free if you really need it). They are very similar functionally, but it can be a bit annoying upgrading from windows 7, based on what I have heard at least.
Either one will work, and I'm almost certain that every post here will be personal opinion. It is really up to you.
Edit: Also note, the 2013 Mac Pro only supports Windows 8. Any system that comes pre-installed with bootcamp 4.0 or above will be hard to install windows 7 on, if it is at all possible. I don't know if the new MBAs come with 4.0, just a fair warning if you are planning on buying a new MBA.
Best,
Matt