Crit does nothing for lava burst or anything under Elemental Mastery, yeah. But even for PVP you want something punchy to follow that up with or you'll be killing the first opponent then be weaker to the others until your cooldowns are up again. So it's still something of a balancing act. But if you have two choices of which one gives much more spell power, it might be worth getting the increased spell power. I would consider that as the main guideline.

For PVE, you just turn your intended rotation into a formula in terms of +dmg & +crit to see what will give you the best damage.