All three of the current M 28 mm Asph lenses are excellent and team well with the Apo-Summicron-M 50 mm Asph. So it's just a matter of size and speed. I agree that for a two-lens outfit for a Leica M camera, 28+50 makes more sense than 24+50, simply due to the viewfinder having framelines for 28 mm but none for 24 mm.
I don't like the Summilux-M 28 mm Asph's weight, odd filter size, and viewfinder obstruction. In terms of performance, however, it will make your jaw drop, especially wide open. But then, the other 28 mm lenses are no worse, just smaller and lighter (and more affordable).
The Elmarit-M 28 mm Asph has the same filter size as the Apo-Summicron-M 50 mm Asph which is no small advantage when actually using filters. Personally, I chose the Summicron-M 28 mm Asph (current version) as the best compromise between size and speed.
... that you won't see in the 28 mm Summicron.
Obviously you haven't tried the current version of the Summicron-M 28 mm Asph yet.
I have the previous Asph version [of the 28 mm Summicron]. Took the new version home for a head-to-head comparison. There is no difference whatsoever in the character of the imaging, and my original lens had better corners than the new one. Needless to say I returned the new one and am very happy with my original lens.
That's strange because the new version of the Summicron-M 28 mm Asph is so much better than the original one. However the field curvature is different, so depending on your subject and your focusing, the corners may seem
less sharp with the new version. That's why I'm always saying: Never try to assess a lens' virtues after just one single test! When focsing properly, the new version's corners at f/2 are sharper than the original version's corners at f/5.6. That said, the original version definitely is no slouch. But the new version is clearly better, and not just at pixel-peeping level.