I would suggest Windows 8.1, it works really fine and handles the retina-display really well. Some must-have apps imo are:
1) MacsFanControl - to set the fans manually as windows doesn't handle them very well (they kick in too slow)
2) StartIsBack - a small app that brings back the startmenu from windows 7, and it does it nativly using files included in windows 8.1 but not used.
3) Newest nvidia-drivers for the 750m, boost the performance alot from the included drivers with bootcamp.
Also, if you do alot of gaming I would suggest ThrottleStop and nvidiaInspector to set the clocks as the computer have some throttling issues. Although the issues are not really solved by anyone yet, these apps tends to make it better with the right settings. I suggest you do a search and read up on the throttling before using these clocking tools.