What exactly do you mean by "should"?
3 time for 6|6 is wayyyyyyyyyy overpowered. 12 time for 15/15 is also pretty darned overpowered.
I agree fully on the unupgraded.
Given the ability of the upgraded, I think 12
for a 15|15 is actually ok.
The reason is that it will be the last creature in your deck so:
1) It will be very hard to get to.
2) Unless the deck is prepared for it in advance, it will only see 1 turn of action
Those two points should be a major factor for balance.
This brings up the next point...
What happens if you have more than one Uroboros or Jormugandr in your deck?