For baling out, height is the main criteria. If at a low height above the water there is a chance you will strike the water without the parachute being fully open, depending on the trajectory of the aircraft and the time it takes to get out of the cockpit.
Assuming there is enough height, I would have thought that ditching is more dangerous, because the impact forces could be high enough to knock the pilot unconscious against the gun-sight just ahead of his head, who will then drown when the aircraft sinks. Spitfires and Hurricanes were not known for floating for very long on the surface.
The procedure for ditching is not straightforward, but the whole aim is to minimize impact forces because these are a square of the speed (i.e. you double the speed you quadruple the forces felt), so its important to fly at the lowest controllable airspeed into wind (to reduce groundspeed) where possible but along the swell/waves so you don't crash headlong into a wave on touchdown. Try also to make the tail-plane the first thing that touches the water because the deceleration won't be as fierce as hitting wings or nose first. Also if its the nose or the wings first, on the Spitfire you could "submarine" (i.e. the flow of water over the wings would take the aircraft down before its even stopped). Undercarriage also must be up to prevent it making the aircraft flip or submarine on touchdown. Canopy open so it doesn't get damaged during the impact and then impossible to open quickly.
For whatever reason, in WW2 they were often feet underwater before the aircraft stopped moving so while you needed to be strapped in to survive the impact, you then had to unstrap pretty quickly and make your way to the surface.
I haven't seen the film, but based on the large number of WW2 biographies I have read, I think that in reality, given it is your own life on the line, and that you most likely haven't parachuted before, its entirely your own personal preference based on your own personal understanding and fears.
My personal view is that if you are doing much over 60mph in a small aircraft at touchdown in a ditching then the outcome is probably not going to be pretty, even 70mph may well make a big difference and 80 or 90 would most probably kill you. I'm not saying you can't survive higher speeds, but if you do then you are very lucky. You only have to read accident reports to understand that if you don't get the speed right, e.g. ditch downwind, or not at the minimum speed, you will be dead. This information only heightens my respect to the people who flew in wartime.