Recently I've got two Inferno Brigades. First has %meter gain, %atk, %def. The second has %meter gain, %hp, %crit dmg. What is best suited for my primed?

Probably the %meter gain, %atk, %def. I suppose it matters a little if you already have defense maxed out, but in most cases %crit dmg is worse than %atk because you don't crit 100% of the time, and even more so because as I understand it %crit dmg can max out, which you'll likely encounter with Primed, while %atk can go as high as it wants.

%Crit rate would be a different matter. You probably want as much of that as you can get on a Primed.

Well, I believe that Crit Rate maxes out at 100%, but that's probably going to be really hard to get to. My point was more +%attack increases your damage by 1% for each +1% atk, while +%crit rate can increase your damage by up to 2% per +1% crit rate, so double the bang for your buck, provided you have enough +%crit damage. That won't happen with most characters, but with a Parasoul with the Critical Mass MA, and a number of tears out, this is actually achievable.

I hope that's clear. A lot more % signs in there than I really like

Anyway, +crit rate and Critical Mass are pretty key to making a Primed that can occasionally one shot people with +100k damage hits.

Crit Rate and Crit Damage max out at 100% and 200% respectively. Also yeah, since Primed usually takes Critical Mass as her MA, you should be looking for ways to abuse that extra crit damage via moves with ATK% and crit rate.