Considering the ease of getting ret-friendly gear as you level up (every warrior-friendly piece is ret-friendly as well), I'd recommend that for soloing. That said,if you can get Reckoning + SoCommand, Prot isn't so bad for leveling in retribution-style (2H vs 1-2 mobs), and feels MUCH less squishy. You can ALWAYS swap to a shield, and abuse holy shield/BoSanctuary when you get unexpected adds.
Ret: Slices through 1-2 mobs at a time like butter. Can die really easily (or burns more cooldowns) if you get 3-5. (Or, that was my experience at levels ~35-40ish.) Befriend a mage for drinks?
Prot: Slow and steady, but You Won't Die. Reckoning + SoCommand procs are a thing of beauty, and allow very mana-efficient grinding. It's slow, and very low-brain-power. I could literally get up and get a drink while fighting one mob, and come back alive. (Not so easy when you are trying to AOE tank them down -- it's much more interactive.) I liked this, as I was juggling a new baby at the time.

I do believe it's slower, though, than either aoe fighting or Ret style.