Depends what your plans are, they both will work on Radial and Bias tires. The 12 bolts are designed to run a higher air pressure (65psi) 8 bolt (30psi)
they will be on bias 39.5 boggers for now untill i step up to something bigger. not gonna be more than 10 - 15 psi. they will be for my full tube chassis buggy that is close to finishing
I you plan on mounting your own tires regularly then go 8 bolt way WAY easier to assemble and disassemble and yes I've owned both. the way the o ring sits on the 12 bout is kinda goofy and can be a pain to get to hold air.