I'm going to be subtle here...
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RETRIBUTION IS BETTER FOR LOW LEVEL TANKING!
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(note: you still tank with a shield and 1hander, don't be that guy who thinks he can tank SM Cathedral with an heirloom arcanite reaper)

Protection becomes viable only when you get Spiritual Attunement (healing->mana), BoS and Holy Shield. And that happens somewhere around level 45. Before that, protection will go oom every other pull. I'm leveling two alts through 20-60 instances right now and I see it all the time: pull -> pull -> MB, repeat.

Retribution on the other hand has (at a very low level):
- Seal of Command, which is a free, 3 target cleave that deals holy damage (threat!).
- Glyph of Seal of Command gives you 8% total mana (base and total mana are the same for Ret, don't gear for int) at every Judgement
- Improved Judgements - more judgements = more mana (see above)
- Benediction reduces mana cost for almost every tanking ability you'll be using by 10%
- upgraded Retribution Aura damage - YES! YOU SHOULD BE USING RET AURA WHILE TANKING! Free, reflective holy damage that generates threat? Yes please, MOAR! Even level 80 tanks don't understand this and insist on using Devotion Aura all the time, even on trash
- Best of all... you can switch back to your trusted 2H weapon while not tanking and do some questing, without being gimped by your tanking spec