Sparks, or crit?

By the way, Sparks isn't for the Sprite. As far as I can tell, it looks like when you 'cast' sparks, it activates as a passive attack. I would say Critical is better because it'll give more damage over time than Sparks would. I'm not 100% certain, though. If it were up to me, I'd get a few points in both to try them out, then max the one I believe is better.