I prefer Spike's Tactical. No specific reasoning, just do.
That said, I would love to build a BCM identical to my ST and test them side by side, but I want to buy a stripped lower and use all of the exact same parts. Too bad BCM does sell stripped lowers.
BCM has the higher reputation as being the most "milspec" of the two if that makes a difference to you.
I don't run anything but BCM.
Period.
Bob