Regarding the units:
1. Imps - simply an awesome unit.
2. Alt-Demons - also good, but not very durable. Good against archers.
3. Hellhounds - Well, what can I say, the upgrade is just super.
4. Demonesses - I still can't get used to the upgrade. They're good for taking out low-initiative neutrals, but in battle, I still prefer the previous version.
5. Mares - A tactically important unit. If Soldier's Luck is maxed out, I take the old upgrade; otherwise, I go for the new one.
6. Lords - This is a very debatable question, depending on how long the game will last.
7. The old upgrade is preferable, as it has more initiative, even if it's weaker. The upgraded version is only good as meat with an ability that is practically useless in battle.