WHY would you purchase a FULL FRAME Pentax? (In point format)

I'm sorry to say, but a 31mm 1.8 is just a 31mm 1.8 on any camera. What I mean is that you won't notice any difference in DOF, just in angle of view. So you will need to get a 50mm 1.8 to see a difference in DOF. Well of course you could get nearer to the subject with a FF, and here the DOF will decrease, but you have to consider that at 31mm it will distort your subject. If doing portraits it might not be that nice (big noses). Still a 31mm on a FF can still be a nice wide angle lens.