About general stats & pierce:
I think you just need to test. It will be easier than doing the math.
Make your alliance, do a 20 stam hit, write down how much damage you did.
Change your alliance, hit again, see if you did more or less damage.
Make sure the health bar is full each time for a more precise comparison. Also if you're using Barbarus, it will increase your damage a bit on the 2nd hit but with 20-60 stam it shouldn't change much.
It really depends on how much pierce you're missing and how big the stats difference is, so we can't tell you just like that. If the stats difference is rather low, then you should probably go with more pierce. If the stats difference is big, maybe not. Just test it.