While I agree that the mouth part is the worst bit, I think it would still be rather ugly helm without it. Bounty hunters lack good looking armours and especially headpieces.
Without taking anything away from the theorycrafting, I'd like to point you to this thread :
http://www.swtor.com/community/showthread.php?t=626183
Especially he comment by shadowlure on the third page is a very interesting and valid one.
Most sensible post in this topic! Since both choices are certainly viable and only theorycrafting supports going all out on defense, I can't but agree with you.