Green gets the savior armor, IIRC, which has the best healing in the game, and I think it can fly, also. It also gets biosuits, arguably the best tanking armor in the game, and the pestulator suit, which has a special attack and is also tanky.
I think green also gets you tamed cyclopses? Cyclopses are great auxiliares since they can strip armor, boost morale and tank rockets with their face, actualy getting them can be really annoying though.
Green has pretty good immediate benefits tho, 3 extra (escaped lunatics?) gals make up for a pretty good early boost.
EDIT: I do agree that there's a large gap in general usefulness between the vessels you get with each codex, though.