Before the 6, I had a fuji 645W (IIRC) that I liked very much, and also the 645 zi. The fixed lens fujis are actually very versatile; my issue was that I often like tight compositions and didn't want to have to get them by cropping into 645... so the option of using the 150 on the 6 appealed greatly to me. Plus square format really made me think in a new way. Anyway, all of the fujis are great fun and the price is right, so check 'em out. But you can forget all of these for closeup work, and for that reason I schlepped my 645 pro out west last time. Glad I did... there were a lot of macro-scale things that I enjoyed photographing. And it's hard to beat the convenience of interchangeable backs.
By the way, you do a lot of instrument photography, right? That being the case I *highly* recommend the 80/4 macro for the mamiya 645 system. Excellent floating element system. Also it's not half bad for normal photography as well; at far focus, the bokeh is a bit unusual but in an interesting/good way, I think.