The SMC-A50/2 will be easier to work with on your Kr. The 'A' means that it has aperture automation -- you can control the f-stop from the camera, for simpler auto-exposure shooting, and flash will be easy to use. It is a VERY COMMON and decent lens but is not considered outstanding.
The Super-Takumar 58/2 requires an adapter, as Adam said. It would not be quite as convenient on your Kr, with no aperture automation. It is also probably not quite as sharp as the A50/2. BUT... it is a VERY RARE and highly-regarded lens, with notable bokeh (out-of-focus area quality). I may even bid on that one! And once you buy a not-terribly-expensive adapter, you are open to a whole world of great (and often cheap) M42 lenses.
Here are the reviews:
A50/2:
https://www.pentaxforums.com/lensreviews/SMC-Pentax-A-50mm-F2-Lens.html
ST58/2:
https://www.pentaxforums.com/lensreviews/Takumar-58mm-F2.html
My thoughts: The A50/2 is a decent working lens; others like it should not be hard to find. The ST58/2 is RARE AS HEN'S TEETH, is worth MUCH MORE, and would be an investment for your future; if you don't like it, you can hold it awhie and re-sell it for a good profit!
Last edited by RioRico; 05-30-2011 at 09:00 AM.