Welcome aboard!
Both cameras are very good, not that much of a difference IMO. Both are rugged, well built, weather sealed cameras.
The K-3 is a bit "more" of everything - higher resolution, higher number of frames per second, a bit faster autofocus, has two card slots, bigger lcd screen, bigger view finder, weighs a bit more. Oh, and it has focus peaking. Not hugely better in any one area but still better in a lot of areas.
So, whether all those improvements will be worth the extra money is hard to say. Depending on what lenses you have it might be wiser to get the K-5iis and get some better lenses. If you already have top notch glass you might get more out of a K-3.
Of course, if you were filthy rich I wouldn't expect you to even ask