For Bokeh and wide-open or near wide-open IQ nothing will beat the A50/1.2 - it wins hands down. That's why it commands the price it does. Just look at the PPG and see for yourself:
PENTAX : Select a PENTAX interchangeable lens camera or a lens model
Personally I prefer the bokeh and overall look from the K50/1.2, which is what I got instead. I realize that some of what I like in the K50/1.2's image quality can technically be labeled as flaws, but I still like the softer look of the bokeh over the A50/1.2.
I think you can actually see the differences on the PPG without needing to do any direct head-to-head testing.
In other words, the best A 50 is the A50/1.2, and the best M is not really an M but the SMC "K" 50/1.2. Amongst all Pentax "50s" the best ones are those two plus the FA43 and the DA*55. Each is uniquely different, however, so it depends on one's needs. The F50/1.4 is a good ways behind those four, but overall (by a small margin) it's the best of the rest.
I hope this last part helps answer your question, even though it may not be a direct answer:
I don't feel the need to do or read any head-to-head testing at this point. I've owned the K50/1.2, F50/1.4, M50/1.4, FA50/1.7, F50/1.7, and A50/1.7. Not a single one was a bad lens, or even mediocre. They were all good. But I've only kept the K50/1.2 and A50/1.7, because I like them better.
In reality my purchase of the FA43 made all but the K50/1.2 obsolete. However, I still love the A50/1.7 and it's such a good value I couldn't see selling it.
Last edited by DSims; 05-19-2012 at 11:40 PM.