Windows 8 vs Windows 7 in Stability

Solution
I just switched to Windows 8, and honestly I can't really tell a difference. No extra crashes or anything, except when my overclocks aren't completely stable lol. :p

I think you are good to go for either honestly.
They should both be mature by now. Windows 8 has been out since 2012, Windows 7 since 2009. Main difference is the user interface. However, with Windows 7, I think since they are now going into extended support essentially, don't expect to new features like new versions of Direct X, etc.