Cost and schedule maybe? I found most shops were quoting 3-4 weeks for cerakote, and up to 8 weeks for nitride. Some charge more for nitride as well. But many tell me the nitride is much tougher than cerakote. I don't mind, really, as it's a Glock carry gun and I am not worried about cosmetics.
I just had my non-MOS black G48 milled for a 507k. I had the slide recoated in graphite black cerakote. It's hard to tell between the original finish and the new; here's a photo of it next to my stock G34 MOS with a 507c mounted on a FCD plate. I think the cerakote on mine now is slightly less slickery than the black nitride (a good thing, as far as I'm concerned). The silver slide G48's I've seen/held seem more rough, I am not real familiar with the silver finish but I understand it's some other kind (PVD?) than black nitride.