I love the Hassy for landscape and some family/friends portrait work. I use a Sigma SD1M for digital landscape as well, flowers/closeup/macro with a 70mm Sigma 2.8 and I have a Pentax k3 with a whole bunch of FF lenses for general low light and photographing grandchildren etc.
So the Pentax 645D is attractive because it could do the close up work at high res with the 120mm FA Macro lens and the portrait stuff as well, plus I could perhaps use my Hassy lenses on it was well.
If I go with the Pentax 645d I will likely sell all the Hassy gear except perhaps the two lenses. (Not sure how they compare to the Pentax glass, it has been hard to get feed back on that front).
So there is some consolidation involved. I probably should sell the Sigma as well because of overlap, but I just love the output of the Sigma sensor, so can't see that going, even though it is somewhat of a specialized and temperamental platform. When it works it produces photos like paintings and I don't want to lose it for that reason.
I had been considering the FF Pentax, but I used to have a 6 x 7 Pentax and loved it, and am drawn to the Medium format world for the landscapes.
Hope that helps.