if you have a choice between armor and helmet you probably want to go with armor, because it does provide you with much greater overall protection. Ya, cool, a helmet prevents a dretch from doing 96 damage with a headshot, but if it hits you in the torso a couple times (much easier) you're just as dead unless you have light armor. Same case with goons, if you cant prevent a goon from getting a headshot through tactical movement (see my guide in the features/articles section of
www.tremulous.info) then chances are pretty good you're definitley not going to be safe against its 1-hit-kill pounce, or 80 torso damage in one chomp. Light armor provides much greater protection agains goons than a helmet does.
What makes helmets more expensive than light armor is not protection but radar. Yes radar is very nice, but its good practice anyway to be able to play without it, keeping an eye open for likely ambushes.
Personally, if i have to choose between one or the other i usually go with neither until i can get enough money for both. But to underestimate the usefullness of light armor alone when worn by people who know what they're doing is to be a very dead alien or a noobish human.