I cant find it now but I once found a good comparison tests of k and screw mount 50mm lenses. Of the K-mount lenses, the 50mm f1.7 was a hair better than the f2 and f1.4. I think the 55mm f1.8 was best in the screw mount group.
-Did you buy the MX? -if not, can you PM me where you found it? I may like to buy it.