Low level spirit better as a mage in impossible?

[Leind]

So far I am sticking with the 20% soul drain at 5 rage for Reaper, with low rest, and when I haven't a rest or a cost decrease upgrade I upgrade the black hole ability. Now that I have reaper I use a lot less the other spirit.

[Calinda]

There are only 2 cost upgrades per skill. Also, they require the skill to be certain lvl before they can pop out.