Polaris has been a pretty prominent X-Factor member since the 90s and still is. So I don't know, think the answer your looking for depnds on the reader. Her and Quicksilver always fit more on that team imo. But she definitely has her fair share of X-Men books with a few major arcs. Magik has been in X-Men books since at least the 80s before she got kidnapped and went New Mutants, she also dies in X-Men sometime in the early 90s. Either are fair game right now is more my point. There's no New Mutants or X-Factor, just X-Men. Nothing wrong with having a preference though.
Again, I think Polaris is a better choice here but not really because of when she was introduced. None of that has ever been remotely relevant in the film series. More so her Apocalypse past connections, plus Havok and Quicksilver being around. I'd prefer they cast for story and relationships. She makes the most sense as of now.