If I were to choose between a piston twin and a PC-12, I would choose the PC-12. I have spent a lot of time in piston twins, including 800 hours in the previously mentioned 421. Im not a piston-basher, because they can make sense in some situations. However, I would trust 1 PT-6 over 2 recips in most cases. As far as a King Air vs. a PC-12, I would think the second engine could become worth every penny. A 200 is no rocket on one engine, but it aint no slouch either. I think a PC-12 is a fine machine, but I think if I had mountains and distances, I would take the King Air.

The problem people get into is buying "less" of an airplane and trying to do more with it. For example, saying a 421 costs less than a King Air, but then trying to make a 421 work like a King Air (6 passengers, luggage, 600 miles).
Vaule is buying the right airplane for the job.