Which do you use on what? If you're fighting another Mage, sometimes Storm Atronachs are best (i use them to bring Dragons down as well). If it's just bandits, the Dremora Lords are good. The main thing is to figure out which Atronach is better suited to fight what.
As far as their health, I really don't know how their health is increased unless it increases with your char. Also, can you dual-cast your Atronachs?