Ok I know this is a little late but here is my two cents worth. I say go with what ever you are comfortable with. I will say this though people get sunburnt scalps all the time that have full heads of hair. So if it were my first time out in the sun fully exposing my body I would use a spray on sunscreen that would not gum up in your "bush" if you decide to keep it.
if your concern is sunburned lower lips, I definitely would think the bush would protect. Though I have female friends who trim theirs with similar results. As they say that unless your nether-hair is fine and not coarse enough to tangle, too long can create a momentary pain when parting the hairs ..