The 90mm f2.8 would be better for its tiny size — it really is a treat; I'm looking for one. There are bigger brutes in the lens line up, with superior optics. The 90mm and 105mm are average by many real-world standards. And the difference between 90mm and 105 is very, very slight. Both lenses focus easier than the several f4 to f4.5 versions in low light. This should be borne in mind if you use a polariser or deep red filter.Pentax 67 + 90mm f2.8 (I prefer wider than the 105mm...)
At the end of the day the format should be the deciding factor. With the Hassy, 6x6 is all you have; the P67 will allow you versatility to crop whatever you please.
PS: Invest in an eyecup if the P67 has a TTL prism meter. The moment you remove your eye from the prism the reading will bounce; there is no AEL (this is an old skool clunker, remember?).