primo for me, masteron had me hard and horny though
I have run mast at dosages of 800mg and the effects were pretty crazy actually. you will put on way more mass than you would ever think, and it is all quality maintainable gains as well. It also promotes the leaning hardening effect if you are in single digit body fat.
however, that being said, primo at a similar dose would contribute to way more in terms of mass, and i believe it to be just as maintainable. I have run primo at 1g a week for 15 weeks, and the gains were awesome. Primo I find to be more mild too in terms of recovery than Masteron.
So really, I agree with the others. For mass? Primo all the way. For a little mass and ultimate hardening pre-contest? Masteron all the way...