Decreasing your recovery time has a better effect on your DPS the lower your recovery time already is, whereas increasing your damage is a flat increase that actually gets less impactful the more damage you already have.

At first, increasing damage does more, but later on lowering recovery time makes a bigger difference.

It works best if you focus on both (especially since the more you increase a skill, the more it requires to increase it further).

Of course, when you hit the recovery time cap only increase spear skill from then on.