Unless:
You want to run a lot of instances
or
You prefer AOE grinding over quests
or
You simply hate Ret.
Go Ret. Most people are in the business of leveling and not experiencing old content - if you want to hit 70, then work on Northrend - Go Ret. Paly tanks can be fun to level, but:
Ret is faster
and
Ret can tank till outlands with ease
and
Ret can tank through outlands if you're good
and
Quests are the best method of getting EXP.