I just got big un and miscreated monster. I'm at 20 on all dungeons, 2 key unm gold 5 arena stage 20 hard mode Doom tower. Who of those two brings more over all long term value to an account?
I would need to see your roster. Both are excellent, but depending on what needs your account might need filled, their value will fluctuate. The answer is likely Big Un', but would need to see to be sure.
Big'Un I use almost exclusively in arena. Pretty much don't use him anywhere else. That said, I don't use MM anywere at all, so ... take that with a grain of salt.
Without any further information I would say you should decide by whoever of those two will bring you more in Faction wars. That's the only content I use both of them...