I've been debating this one myself for some time Woody, and haven't fully decided yet.
The 60 is tempting, but one point that I haven't decided on yet is that the H4D/50's native ISO is 50 where the H4D/60's is 100. Sounds like a small issue, but having used a Leaf Shutter H camera with an ISO 100 base (H3D-II/31), there were times I wished for 50 ... and ISO 50 is what I always use in the studio with strobes. Both top out at ISO 800 and have the same capture rate.
With the newest version of Phocus and the latest firmware, the H3D-II/39 can do 64 second exposures. Hassey lists the H4D/50 at 32 seconds ... and it remains to be seen if they goose that up like they did with the 39 recently. Hassey hasn't published the long exposure specs for the H4D/60 yet (that I can find).
BTW, I don't think use of the HCD lenses reduces the 60 to 50 ... the crop factor is minimal as I understand it, and I'd guess the 60 becomes a 55 or so (that would be splitting the difference between the two sensor sizes). No clue how the edge performance will be with the HCD lenses, but I'd suspect DAC corrections for some of it if there are correctable issues.
I haven't used the later versions of a Dalsa sensor ... my last experience was with my 33 meg Leaf Aptus 75s. So it is hard to comment on. I do know that the Hassey 50 has received good reviews compared to the Kodak 39 in terms of cleaner, more natural color rendition and great skin tones, so it's hard to use any Phase experiences here because there is no Phase One 50 if I'm not mistaken.
What recently caught my eye is that Hassey included the H4D/50 Multi-Shot in their trade Program ... which is only $3,000. more compared to trading a H3D/39 for a H4D/60. That, more than anything is where my debate lies since a H4D/50 Multi Shot is probably the highest IQ camera they make for certain applications ... while still providing single shot 50 capture.
-Marc