Dragonstone Pendant or Rainbow Chalice?

So I have just reached 80/80 for Dragonspire's pity summons. I also have 2 of the 3 5* dragons so the choice is really down to these two. I have 1 of each in my inventory, and plenty of 5* dragons at 3-chevrons.

Which is more likely to be the bottleneck for a fully ftp player? Judging by the historical account, I think both are equally likely to be dropped and used.