Finn wanted to save Rey and get away from the war. It was a selfish reason. Rose saved Finn because he's become one of the best Resistance soldiers, and him sacrificing himself on a gambit that wasn't likely to succeed (like everything else in this movie) would've been a waste of a good soldier and a close friend. She knew that the Resistance needed as many allies as they could get, not to fight today, but for the chance to fight another day.
At that point, that was the best chance that they had. If Luke didn't show up they would all have been killed. And it was no way that could have been predicted. Hell what she did should have gotten them both killed. BTW did you noticed how Rose main goal was to take away Finn's agncey . ---
Bender: Well, everybody, I just saved a turtle. What have you done with your lives?