[2020] Should i use B Grade stamina gear or C Grade critical hit?

Until you hit at least GS 1260, gear score is absolutely the most important thing. Try to pay attention to stats on the gear too, but when push comes to shove higher GS wins

A is not always better than B. I wouldn't give up attack power for soul pyre regen or something else useless. You kind of have to play that one by ear. When you're at the stage where you're swapping B for A gear, your main priority is making sure your gear score is high enough to get you into abyssals so you can get the A-drops you actually need. If you get an A-sword with a gathering bonus that still has more attack and GS than your B-grade weapon, then its good to switch to that one. But if it takes you down a rung AND has a bad effect, better keep the B weapon