I think Kylo works better, because Ren is too close to Rey, and also are all the Knights of Ren called So-And-So Ren? Because that could get confusing, to refer to any of them as Ren.

Buuuuut in the novelisation, he's referred to as Ren. So when I'm writing fic, especially anything from the perspective of somebody in the First Order, I call him Ren to match that canon feel.

So basically, Kylo is my preference, Ren is canon-typical, and I will accept either when I'm reading fic. :3