AFAIK, the Crit doesn't work on Mercs, so that's a non-issue.
The AR is also pretty much a non-issue.
That leaves 17/221 CV vs. 16/259 CA:
- CV has a really high str req, which limits your other gear choices
- slvl 17 Medi is not going to be noticeably better than slvl 16 Medi
- the CA will obviously do more damage than the CV (approx. 55 more avg weapon dmg)
My vote goes for the CA. If you're really that caught up on the 17 vs. 16 Medi, just reroll the CA 'til you get 17.