Spell damage doesn't exist, far as I know. Tooltip damage is how much damage an ability deals, before factoring in the target's calculations.
Spell Power is a stat that is factored into the equation.
You should focus on both. Set bonuses that increase spell damage or weapon damage increase tooltip values more than
those that provide max resource bonuses, so focus on spell damage through sets.
There are a lot of passives that grant percentage increases to max magicka, however, so with regard to endgame builds, there are definitely times when max magicka set bonuses come close, and might be the better way to go.