I am a bit odd in my gear purchase. I generally bought too many and too soon and made a lot of mistakes. So you see my background and please read my inputs with a grain (or a jar) of salt.
I have both 31 and 77 at one point with my K10D. I sold the 31mm after a month. I found it too expensive to own and I didn't feel comfortable in the ownership. It is like buying an overpriced expensive car and I can only admire in the garage. I know I exaggerate but that was how I felt when I sold the lens. It was one of the best lens from Pentax but in the end, I chose the other limited and gave up for the 31 -- my LadyQueen which needs to go in a diet as I prefer something either wider in view, leaner in size or something that serves closer as a normal lens in the cropped view.
Do I miss the lens? Yes, maybe a bit. Had I kept it longer till now, I would have made a small fortune as the used lens was about $690 at the time. In hindsight to many, it was a stupid decision. To me, it was the right thing to do.
The 77mm is a different story as I bought it for portrait purpose. It is one of the insanely good Pentax lens for ownership. If you don't care about FF, the DA 70mm f/2.4 limited can be just as good only half a stop slower.
I replaced the 31mm, my LadyQueen, with Pentax DA 35mm f/2.8. It is a different lens for different purpose. And it is my best documentary lens. It is not as fast but its use is much more in my bag as I use it in traveling as a normal lens, flowers close up, many product shots in my blog page that the 31mm will not cover but left in the garage idle for appreciation.