Casting Speed vs. Spell damage

Fancy calculations aside I think it's best to take as much cast speed as your mana regeneration/leech can support since it's less accessible than spell damage and you're probably helping the ratio by investing into it more. Cast speed also gives the less obvious benefit of improving your ability to kite since it reduces the lag before and after casting a spell until you can move again.

For totems I guess my point is moot though, unless you value casting your totems quickly.