Depends a lot on your playstyle - Corrin is much tankier physically, and cordelia is a glass cannon - maybe too much with those IVs. On the other side, Triangle adept is very much a blessing under the current meta.
The safer bet is probably to promote Corrin - At 37 Spd and 34 Def she can work quite well as a damage dealer and as a tank. But again, depends on what you want either for. Also, Ninian has Ward Dragons, which can be interesting with Corrin. But you'll need something to deal with high Res characters.
Thank you for your answer. Yeah, I think it will be Corrin for now, I mean, I have many that I intend to promote eventually; but since I'll probably need to use Ninian in the arena this week (not that it's a problem, I quite like her), Corrin will help me more.
As for how to deal with high res characters, I have Lucina, Takumi, etc, so no problem. Thanks again.