Windows 7 vs. 8 on a Mid 2012 MBP?

Windows 8. It's slightly faster in normal operations than Windows 7, but the real reason to get 8 (since you don't mind the UI) is to ensure a longer period of support. Windows 7 will be supported by Microsoft for the near future, but Windows 8 will be supported even longer.

For what it's worth... I have a mid-2012 13" cMBP and Windows 8 runs spectacularly on it.

 

If you need to run anything out of the ordinary, check first. In a course I teach a program that we use is not yet supported on Windows 8, and indeed it doesn't work. Professional software may move slowly since most businesses are doing a wait and see.

Microsoft is just dropping support for Windows XP now, so I'm sure Windows 7 will still be supported for well beyond your school years!

 

Windows 7 will continue with full, "mainstream" support until January 2015 then go to extended support (where XP is now) until 2019. To see the difference between the two, MS provides a chart here.

In essence, you have nothing to worry about in terms of Windows 7 going unsupported. I have a feeling it's going to be this decade's XP, where the extended support phase might very well go past what MS is targeting right now.