600 mgs of primo is fine, especially with 200-250 mgs of test. It doesnt hit hard or fast, but is clean, low sides, and will work at that dose. I have known people who run it at 400 and do well. It is not going to add mass amounts of weight or strength. I love primo as it is easier to recover, which I know you dont care about, has very low sides, and will definitely enhance your physique. I have used it at both 400 and 600 and it worked well at both, obviously better at 600, but to say anything less is a waste based on my experience is untrue.
Mast is available in the same ester, enanthate as primobolan, and mast is available as dipropionate ester, and that one will yield more hormone mg per mg, but the enanthate version will be the same. Whether mast is more powerful than primo mg per mg is debatable, they are close and I think it is personal preference. the nice thing with mast is that it is available in short ester so gains are usually noticeable quicker, and cycles can be shorter, but in a longer cycle that is not necessarily a great benefit.
I say use one this cycle, and the other next and find which you prefer,