What makes more sense? A fancy new full frame wide angle lens? or a cheaper wide angle?

The FF Pentax debate has been going on for years now, rattling the foundations from time to time!

Really , the best reason for buying into Pentax are for its small compact prime lenses, especially the "Limited" range. Never say never, but my advice would be to shelve the need for FF compatability and aim more for the angle you hanker with the camera you have.