Which alternate upgrade for Inferno do you like the most?

Old Imps are, in my opinion, just as good as new ones.

No, they are not equal... think about it:
Old Imps drink mana without spending a turn, and that's a huge plus!!! With them, you can drink mana and summon reinforcements in one turn...
New Imps need to spend a turn to drink mana, which means a smart mage will be able to take them out before they summon their brethren to help.