I own 21/24/28/35/50/75/90/135mm for Leica M - but my usual picks are:
- 35mm, most often, the unsurpassable Zeiss Biogon C, but also the new Voigtländer Ultron f/2 and the Nokton f/1.4 II - i.e. compact lenses; I like the Ultron f/1.7, but find it a tad bigger than I prefer my RF lenses to be - of course, YMMV. Yes, I do own lots of RF 35mm lenses ... I haven't listed them all here.
- 50mm, most often, the Voigtländer 50mm f/1.5 "1" and the Summicron-M 50mm f/2, but also the quite wonderful Elmar-M 50mm f/2.8. Again, I like my RF lenses compact.
- 75mm - the Summarit-M 75mm f/2.4; it's all I'll ever need at this focal length.
More to the point, I've travelled with 28/35/50/90mm: the Elmarit-M 28mm f/2.8 (pre-ASPH), the original Nokton 35mm f/1.4, the Summicron-M 50mm f/2 and the Elmarit-M 90mm f/2.8. The combo worked out pretty well, but I actually struggled a bit with the 28mm (on the M10) - I'll opt for 24mm next time (I am privileged to have acquired a quite wonderful Elmarit-M 24mm f/2.8 ASPH in the meantime) and take two 35mm lenses: my Biogon C that I sorely missed last time and something faster, most probably the Nokton f/1.4 II this time (because it's slightly less capricious than its predecessor). There's even a slight possibility I'd swap the 90mm for the Elmar-M 135mm f/4 - because I really dig that lens, but of course, it's even harder to work with than the 90mm, and a bit bigger ...
Back on topic: The Voigtländer Ultron 28mm f/2 lives on the M8 - I've used it on the M 262 as well and quite liked the results, but I have to admit I struggled a bit with seeing the frame lines on the 262 (I wear glasses, after all). 28mm works better on the M10, but not exactly comfortably so. For anything wider, I use an external viewfinder anyway; I own a somewhat bulky Leica 12013 viewfinder: 21/24/28mm switchable, big, but gorgeous to look through, and it actually works quite well for framing.
Anyhow, the Ultron is nice lens to work with, small enough, good enough - not stellar, but reliable. This isn't a Summicron, nor is it an Elmarit - but it's still a solid lens. And on the M8, I clearly prefer it over the other option I use regularily, the original Nokton 35mm f/1.4 (both with IR/UV cut filters) - in spite of the latter's vintage rendering. The Ultron is just modern enough to rarely surprise you, but classic enough as to not be too much "in your face".
The takeaway? You have to want and like the 28mm focal length - but even if you do, the market isn't that rich, especially compared to 35mm options. The difference in the FoV is tangible, but only you can say whether it's important to you. To me, it's kind of an in-between - I usually prefer 35mm or else, an even wider view (24mm works very well for me, and I've found 21mm and 20mm to be good choices for me, too).
That said, 28mm lenses with smooth transition rendering are wonderfully versatile tools, and I know that my Elmarit-M and my Ultron both work well that way; both aren't the smallest, and while the Elmarit-M is pleasingly sharp wide open (not bitingly so, but certainly good enough), the Ultron is slightly soft on 24MP FF sensors; both are at their best at f/5.6 or f/8. So, for street and documentary, specifically people-centric shooting, both are great options, and I'm sure the sharper, smaller, yet a bit more capricious (vignetting, falloff, bokeh) Elmarit-M ASPH will be great for this, too. I'd be a bit wary about using any 28mm lens as my only wide option for landscape and architecture, though - the Elmarit-M 24mm f/2.8 ASPH is a better choice for that, even if I have to also carry the additional finder. But good 24mm RF lenses are even rarer than 28mm ones ...
M.