Quote Originally Posted by sanahtlig View Post
My guess is that drops are dependent solely on number of enemies killed. If I remember correctly, sub-quest #2 (Diabolos's Surprise) had 3 waves of 3, which would maximize kills.
According to JP Wiki, that one has 8 mobs. Plus a chance of 3 mobs turning into a Rare one. The Free Quest Nike's Rampage has 8 mobs as well, and doesn't have a rare mob.

Rare mobs tend to give R weapons for fodder, but less stuff for Vine. So up to you to decide which one to farm.