Its all about personal prefs, but If I would do this, I would go for a subtle and light darkening, so the contrast with clear glass would not be too obvious.
I would worry much more on the "how" to do it and how long it will look good.
Using films, as the window is exposed in sunlight the whole time, I would be afraid to see bubbles appear after a while or get discoloured regions. Even if a "uv resistant" film is used, the sun isn't always complying to manufacturers wishes in the long run.
With spray, the spotless glass has to be flat lower than spray gun and to apply the layer evenly distributed is very tricky. Also you have to be careful not to get any scratches when you put it back and after that.
If you go for it, I'm looking forward to read how you will do this.
I have contemplated this for a long time. Thats a very big window. I dont think it will look that odd but it will be nice to see it first.
My friend that knows something about film told me that due to the width of the window you will need two sections of film. Therefore you will need someone extremely skillful so it looks good. Thats what is holding me back.