I went for the drop increase, which I'm pretty sure is increasing the number of double/triple/etc coin drops per monster. So it sorta acts like a gold booster. I also seem to be obtaining a much larger amount of shards than I would get from rebirthing at 500. Though interestingly, I have yet to receive a single fantasy shard, wonder if its a bug (meanwhile I have 3 fires, 2 waters, and 2 electric at stage 3 at the minimum)...
Also, a trap I've noticed. Panda and Monster's level 3 skill are....really freaking low compared to what it should be. It's almost a 50% drop in dps from the level 1 skill.