Which is better?

So I have two level 150 soul shooters (angelic burster weapons). One is the rief white worm with 337 attk (yes I know it's low) and I have a magic grade piu soul shooter with 321 attk but plus 9 crit attk rate. Which do I keep?

The 9 crit attack rate should do more damage, especially if you have a high crit rate.

9 is very high. Its a shame the attack is a bit low.

Does 9 crit attack mean i hit higher when i do crit damage or does it just mean theres a higher chance? because almost all of my shots are criticals already.

Critical rate refers to the probability.
Critical attack rate refers to the multiplier.

The base multiplier is 2x normal damage.
Your weapon will improve this to 2*1.09 = 2.18

Alright thanks.