Should I grail Gilgamesh or Cu Alter? (NA)

So, i have fully ascended both my Gilgamesh and my Cu Alter. I know a lot of people say that grailing Cu is the way to go because it benefits him a lot, but there's also the saying of "grail your favourites" and Gilgamesh is definitely my most favourite. I currently have 5 grails, so I could either try to split them between Gil and Cu, or I could just pour all of them into one of the two and have them reach level 100. I know it's mostly down to preference, but what would you do if you had to choose between grailing one of these two?