Upped is now quite a bit stronger than the unupped. Not sure if OP though, perhaps the unupped is UP.
What about changing the unupgraded stats to 1|8? Then the unupped wouldn't be UP and it would be more clear whether the unupped was UP or the upped was OP. Moreover, I noticed that the Doom Tank can't use it's skill on the first turn, so with 6 HP it would only cast twice its ability.
Don't forget, however, that purify can always be used to remove the poison counters.
If you make the other advantages potent enough this will essentially just become a forced combo.
So in a sense, the most cost benefit you can gain out of having poison counters is 1 to 2
+ 1 extra card advantage (~2
value... though this is sometimes debated)
So in all, a starting poison count is worth at most 3
upped and 4
unupped.
So going to total quanta value of the card, you get:
unupped: 6
(cost) + 4
(poison count) = 10
for a 1|8 with the doom ability
upped: 6
(cost) + 2
(upgrade) + 3
(poison count) = 11
for a 1|6 with doom and voodoo
I'll leave the assigning of cost-theory value to att|hp and the doom and voodoo abilities to others, but this should be a good starting point:
Is +2 hp and voodoo worth a single
?