Hm, guys, I think the TO is experienced enough not to bothered by such considerations as "much better" or "great image quality", since I would guess he knows something about the little Distagon 28/2.8 or the Leica II for that matter since the former was the forum lens on this board for a long time, something like the entry to alt WA, and the latter the one that had a pretty conclusive comparison thread with the former. Not sure it gets much better than the D28 for that matter ( it gets better, sure, mine is not sharp up to the corner by f5.6, so a Distagon ZM18 will be better if you like to use a crop camera, but it is not "much better" fwiw ).
Mark, my guess, from what I have seen, is that none of the 28 M-mount solutions is a great performer on the A7 at infinity. Depending on your application some lenses might work, some might not. But your aim to go with only one mount will be a difficult task indeed.