I would definitely read the Isaac one. This is gonna sound crazy, hear me out. I was never a huge fan of Derek. But! Judging by all the Derek fics I’m very sure people would love it.
But also I know how time consuming and how quick and easy writers block can happen, so if you think you can do that without burning yourself out then I’d say go for it. Maybe you could start with one, see how well it does and then bring up the other fic again? Or just do both of them and if one does better than the other at least you still have one of the going for you. I hope this helps! I don’t feel like it did but I tried my best.