i have a video coming out on comparing the two, which are quite similar... mast is the best option for aesthetics... muscle hardening, sculpting etc... it has anti estrogen properties as well... primo will build more lean muscle and provide more endurance as well as hardening but not to the extent of masteron... primo has more versatility but masteron is has the most profound effect in terms of muscle hardening, than damn near any other compound.. .you cannot go wrong with either one