Phew.I got all four endings.(So spoilers below!!)
Comparing the two good endings. I think Quinn's route is more like warm,while Zethe's route is sooooo hot(and sweet lol) and by warm and hot, I mean the degree of their love(one meaning of the two words) and also the way they treat each other(the other meaning lol).That's also the difference between many BxG amd BxB games(so personally I like BxB games better XD)
And about the two bad endings, Quinn's route is just sad but Zethe's route feels more like desperate.Zethe's dying for Yuki and Quinn's taking cate of Yuki's mother and then rewinding the second time both impressed me. I think this can also show their different love. ( Or is that simply Quinn doesn't know how to take the spell's recoil? )
All I said above is to express one thing- this game is sooooooo nice and relaxing!! I love this XD and I appreciate the artwork very much as well. So hard to find games like this.