I picked up a P18 cheap, and I'm selling it. That said, the question should come down to the boat condition. Used Hobie parts are easier to find and therefore should be cheaper. Both boats sail well and are pretty sturdy. The TheMightyHobie18 centerboards can be a pain for recreational sailors, but make it easier to turn. The P18 is harder to tack. If it has solid hulls with no soft spots and good sails, you will be happy with it. If you have to replace hulls or sails it will cost you. If you're racing, there are usually many more H18s.
Good luck.