I don't think we have official numbers on what +20 crit severity actually means. I somehow doubt it really is 20% extra damage to your crit, especially since you can get doubles. A weapon that gives 40% extra damage on top of the crit damage..? That's a bit OP
Anyway, I usually go for the crit chance weapons. Accuracy only means anything against fighters or if you are firing cannons at around 10km.