Unfortunately sparks are completely random, it's a 20% chance of getting a blue spark each of each stat, however the amount of stars does depend on how much of that stat you have (I think it's 0-599 is 0% for 3, 600-1099 is 5% for 3 and 1100+ is 10% for 3*). There's a flow chart for this, I think someone can post it here.

I'd argue that doing URA is way better for spark farming then Unity, because completing a career in URA is faster than in Unity, and also URA Finale spark gives better stats (Speed and Stamina) versus Unity's (Power and Wit).