Do the stainless 75B's have the same beaver tails as the SP-01? I could have sworn I read that somewhere.
The only difference between the 18-round SP-01 mags and the 16-round 75 mags are the baseplate. You can spend $5 to put an SP-01 base plate on a 75 mag, and now you've got an 18-rounder. Personally, I prefer the 16-rounder with a rubber base pad, but I do keep a spare 18-round mag in the nightstand, just in case of super duper emergencies where I might need 34 rounds.
Anyway, I voted for ( and bought) the SP-01 entirely because of the rail, but prefer the looks of the 75B. And as I said earlier, I expect to have both sometime soon. Which, this being a gun forum, is inevitably going to be the consensus answer.