Personally I always pick Charm, that skill is very useful. Especially when you charm the healer of a group, not only will he stop healing his buddies, but he'll heal you instead! I think on Nightmare difficulty, that's something you really need!
Blind is useful too, makes an enemy totally idle for a while; if you have both this and Charm, you can make any group of enemies a lot weaker.
Also, if I may, I would recommend you first finish the game on Nightmare difficulty,
without Hardcore rules: it will already be quite a feat, and that way if you want to try Hardcore later, you'll know what strategies to use against some very tough bosses like Amdusias. Otherwise I'm afraid you're going to replay Broken Valley 15 times before you manage to beat Amdusias :p
But it's up to you!
