Hi Gary,
nice to see another ML collector here!
I have both the ML 24mm and the ML 21mm, and it took me quite a while to hunt them down. I think both are very good, you can find some sample images in my flickr albums (see my sig) or in our german discussion forum (
ML 21mm thread and
ML 24mm thread).
What camera are you using? On a crop DSLR, the 24mm will have only moderate wide angle aspect, so you might prefer the extra 3mm. OTOH, the 21mm shows up even less frequently than the 24mm and fetches 2-3 times the price.
Someone has measured the exact focal lengths of the Zeiss Distagon 25/2.8 and the ML 24/2.8 and it turned out that the Zeiss is actually almost a 26mm. Not a big difference to 28mm - I would recommend the Yashica here. The ususal price here seems to be ~ 100€ recently, and I think that's very fair.