25 Vs 45

25 because it's the best compromise if I can have only one fixed FL and I have to choose only between the two specific alternatives you mention.

But like others, I have difficulties with the premise of your question. Having and carrying only one fixed FL doesn't make any sense to me. Why restrict yourself like that, especially if you have bought a camera that allows you to switch lenses?

And if I can have more than one fast prime, it would be the ones I have: 12/2, 20/1.7, 45/1.8, and 75/1.8. In spite of selecting it as the best compromise, the 25 mm FL (50 mm EFL) doesn't suit my shooting style very well (a bit too narrow for "scenes", a bit too wide for "details"), and I usually prefer to go either slightly wider (17 or 20) or slightly more narrow (45).