Why choose to ditch in the sea over bailing out?
Bailing out the cockpit during that era was neither an easy nor always a successful task. The pilot had to either roll the plane, open the canopy, and release themselves to the void, or if rolling wasn't possible, walk on the wing and jump.

That's inherently unsafe. Don't forget that you are not exiting a healthy plane. There might be smoke, debris, fire, propellers spinning, the tail that might cross your way out.

Read what happened to Hans-Joachim Marseille when he tried to bail out. Quote from Wikipedia:


  the left side of his chest striking the vertical stabiliser of his fighter, either killing him instantly or rendering him unconscious to the point that he could not deploy his parachute.


Now, regarding ditching versus bailing out, I don't have any source to substantiate the chances of survival, because in the end, what matters is to get out of there alive. But I guess it depends on many factors, one of them being the sea state as they mention in the film.

Another one is the type of damage the airframe has sustained. It might be better to risk a "gentle" contact with the water if there is no "urgent reason" to leave the plane and you can ditch reasonably soon, than to exit the cockpit and increase the chance of being injured by anything that I've mentioned in the first paragraph.

Finally, it turns out that the choice heavily depends on the situation. I haven't seen the movie so besides the "sea state" I don't know the details. So provided that you are above a water mass capable of supporting the ditching idea, the factors mostly influencing the decision are:


Airframe condition
Altitude below you as Pete pointed out and Philip Johnson mentioned in his answer. Too low and you might not have time to deploy parachute.
Sea state
Time of day. You might want to touch water at a lower speed if you can't see it.
Position of allied forces and/or shore. Surviving the crash is one thing; being rescued a whole other.