If i have to move one of the X-Factor to the X-Men it would be M.

I'm not quite sure how well M would do in X-force,  
Anyway arn't strong-guy and shatterstar the only ones that haven't been on an X affiliated team yet 
 
Rictor - X-Ternals 
Wolfsbane - New Mutants  
Madrox - X-Corporation Paris 
Siryn - X-Corporation Paris 
M - Generation X and X-Corporation Paris 
 
Actually I almost forgot Leyla Miller. She has my vote, her and Emma on a team would be hilarious 

Going from X-Factor to the X-Men is a step down.

I think Shatterstar can re-join X-Force.

I would LOVE to see Monet on the X-Men, especially alongside Jubilee.

I think Monet is a good character all round that could get a decent popularity boost by being put in a main X-Men title, and also benefit the book, and other characters she might work alongside.  
 
Shatterstar and Rictor I see as a package deal, I think they would be good in X-Force for natural reasons (history), or X-Force type team (not so much the current one though) 
 
Longshot could bring back a lot of fun into a main X-Book, he does in X-Factor, but ideally the writer would be familiar with the last time Longshot was in the main X-Book.  
 
Madrox I think, needs too much panel time to work as well in a bigger book, unless he was the star. He's a great main character of a team, so I think works better in a smaller book.