Hands down combat is the way to go until 50 for leveling. You have multiple cooldowns in case you get adds, you do decent damage, and you really have no alternatives besides Subtlety (which isn't really a good alternative at all, as all of its skills will support your openers... and that's it).
Here's a sample combat build at level 40: http://www.wowhead.com/?talent#fZMxAd0cxodz
You can vary it a bit, changing points out of lightning reflexes and into the specialization of your choice (swords is generally considered the best). My next points would go into Blade Twisting, then Vitality. I didn't find Sword Spec all that amazing, even with two BoA swords.
As for PvP, this build gives you more health with Endurance. You might also think of taking Imrpoved Sprint/Improved Kick, though I consider it was a waste. I'm not a PvPer, though.
At 50 Mutilate is amazing for leveling; however, I imagine it would be worse for PvP because it's all about getting the element of surprise and bursting down your opponent. Then again, since there's not much resilience until 60, it might work ok. But you'd be a one-hit wonder: if you get the jump your opponent is probably a goner, but if they jump you or bring friends, you might be taking a spirit walk back. Then again, just keep your cooldowns handy; evasion, blind, and vanish all work wonders in PvP OR PvE.