1 piece vs 2 piece scope mount

clearence is irrelevant with mag fed rifles and the weight dif is so negligible is isn't worth mentioning. One piece base allow for multitude of mounting ring spacing and various MOA of angled mount for long range shooting etc.
 
Nightforce sells two piece base sets that have MOA angles. If you have a top loader, a two piece set is the way to go because of the access it provides you to the action port.