I have a K-11. It has a longer throat and is easier to load cast bullets in it. Cast bullets are usually longer and heavier than jacketed. The shorter throated K-31 is not as easy to come up with a cast bullet load because of the short throat. You going to shoot factory or reload with jacketed...get the K-11 or K-31. Plan on shooting cast...get the 20% discounted K-11.
I love the way the K-11 bolt glides back and forth like it was running on greased ball bearings. Them Swiss knew how to build watches and rifles.
The K-31 is just redesigned to be a bit shorter in the action. I still prefer the K-11 , it seems to have a smoother operation. Try them both and see which action feels best.