As long as you plan on getting the M4 ramps cut, I would go with the Stag. if you are not willing to get the ramps cut, I woudl suggest an alternate upper which does utilyze the correct twist, chamber, and M4 cuts.
http://68forums.com/forums/showthread.php?t=5609
The best advice I can give is to check that thread and choose an option receives an A grade