Can't see why the rewards should be different really.

For sure, the time vs exp reward is proportional between the 2 dungeons but when it comes to effort vs exp reward, it is backwards.

I personally feel a good chunk of those who leave are there via mentor roulette and therefore the acquirement of exp is irrelevant to them and only care about the time it takes. I'd wager they would probably leave if they got Praetorium too.