Get 'em both. Honestly, I think the K-31's are better rifles, but the K-11's are scarcer, and will probably rise in value quicker.
I have four K-31's. They used to be 80 bucks, someday they'll be 500. Compared to other milsurps from the same era, there weren't many made (maybe 500K?). They're extremely well made.
Go to a vintage rifle match and watch what most people shoot. You'll see a lot of K-31's. If only they had sights like a Springfield, there's no telling what you could do with them.
I can do 1 to 2 inch groups at 100yards all day long, even with the sliding tangent sights.
One of mine was really rough, covered in rust, stock was cracked. I sporterized it (calm down, purists- this one was really rough). I removed the handguard and cut the fore end of the stock back a few inches. This completely free-floated the barrel, since the stock is already channeled out all the way to the end, and only touches the barrel there because of the bayonet mount.
I mounted a scope, and I can easily keep GP11 ammo within MOA. When the moon is right, I can do .5 inch groups at 100. Amazing rifles, for a milsurp.
GP11 is a little pricey by surplus standards, but it's very, very well made and extremely accurate. Handloaders can maybe do better, but commercial loads won't touch it. Hornady's SP hunting round won't shoot nearly as well as GP11.