Well, if this Death or Balance wizard is going to be separate from the Life Wizard, why not have both schools? One as main and one as secondary.
And if one of these is going to be the secondary school to the Life Wizard, I would pick Balance in my opinion. Because although Death and Life are opposites, they still have sort of the same, I don't know how to put it, strengths? Death can heal a little also, so you don't want to much of the same quality, at least in my opinion, so I think Balance. It brings in a different strength that Life doesn't have already.
It is completely up to you though, I was just putting my two cents in.
I hope you find my input somewhat helpful in finding the perfect school for you!
Hope I could help, Allison NightGem - Lvl 35 Thaumaturge Destiney NightGem - Lvl 19 Thaumaturge