I always love it when people ask this question. As many have pointed out, it doesn't really matter what spec you play while leveling. I would ask you what you plan to play when you get max level instead.
BM: great solo spec, able to tame creatures with raid buff abilities. Out of all specs, BM is the lowest overall DPS. It is last in AoE damage, and second in single target.
MM: Raid spec/PvP spec. This spec is king of single target fights and second in AoE. With resourcefulness, silence and barrage it is easy to see why this spec is used in arenas and BG.
SV: Hands down AoE. At lower gear levels this spec gives you the most bang for your buck. It is last on single target but tops AoE. It has a lot of useful tools like entrapment and wyvern sting that allows you to control adds.
What is you purpose in playing your hunter? While I firmly believe you need to know all trees of your spec, I also believe in practice makes perfect. If you can see yourself in one of the above playstyles, level in that tree. It makes you a learn those mechanics and priorities and makes you a better hunter.