both......................
primarily DHT of course, but body hairs have androgen receptors which can uptake other male hormones like testosterone, andro, DHEA, etc. DHT, made in the outer root sheath of hair follicles from testosterone, has about five times the affinity for the androgen receptor though (like a magnet and steel if you will), therefore when its made, it often binds. When DHT binds, it seems to bind pretty solidly and the DNA within the follicle reads it and follows preprogrammed instructions. In body hair this instruction is apparantly "grow hair" and the dermal papilla releases growth factors to the other parts of the follicle like the keratinocytes, root sheath, epilitihial cells, etc.
In head hair over time, the opposite begins to occur.
Docj077 has pointed out that alot of men with male pattern baldness have certain variants of the androgen receptor making it more active or in greater prevalence in their hairs. This might account for the many "hairy men" who also have alot of body hair. However, some smooth men also have baldness, so its more complicated than just that as Doctor will tell you.
Overall, being a really hairy dude isnt a good sign for your head hair's future, but it doesn't mean baldness is absolutely going to happen because there are plenty of hairy men who dont bald, but more often than not,,,,,,,,,,,,,,,,,they seem to.