If you're going to quest in Prot, go all prot gear.
A lot of your damage from the Prot Tree will come things like Block Value, Strength, etc.
The whole style of leveling as Prot, will be to pull multiple mobs at once, which your tanking gear will help with.
If you're doing prot, you *will* be using a 1h weapon + shield (please, save yourself some embarrassment and don't stick a 2h weapon on in Prot!), so if you're doing that, you might as well don your tank gear too.
If you happen to have a ret set just laying around, then you could stick a couple +crit pieces on, but I wouldn't sacrifice anything with +block value IMO; your main single-target DPS attack relies upon it.