It's mostly the subject that dictates the focal length. The wider the lens, the rarer the occasion to use it with meaningful results. That's IMO.
I have a Voigtländer 15mm f/4.5 and it takes a lot of time to end a roll.
Wide angles do pose problems with architectural subjects, it's very easy that the subject becomes distorted in many ways which are not immediately visible when you take the picture. An horizon which is not straight will be immediately visible, and if the film plane is not really parallel in the left-right plane to your subject you'll see a strange "lateral convergence" in your picture (same effect as so called "prospective distortion", but in left-right direction which really looks visually wrong normally).
Besides, with very wide angles you'll find plenty of incongruous and undesirable elements in your composition: what's in front of you is the picture, and also what's outside of your visual field if you use a 15mm (!).
Personally I would have looked for a wide-zoom for the reflex (wider at 24mm or even at 21mm), and some wider lens for the RF (an 18mm or maybe a 15mm) BUT the external viewfinder must be in axis with the lens, for the reason above explained.
Wide angles are a bit "difficult to use" for the care they need and I suggest making friend with them with some graduality, or you'll risk to curse them very fast