Phil,
I recently bought H3D - 22 which is similar to P25, and although I still have have some problems with the H3 body (hopefully a new one will be send to me) this was one of the best decision I recently made in my photography. I had a 1DsMk2 for a year the and the files were o.k. but I was always unhappy with it, I was missing the days of Pentax 67, Mamiya RZ, I really didn't feel like myself anymore. Also I found that my style of photography had changed.
I think the 2 files shouldn't be compered just in terms of sharpens where canon is also good. The thing is that image from a back feels totally different. The difference is really huge and that's also why I sold my 1DsMk2, and I'm happy I did so. However I shoot mostly fashion and advertising and the speed of H3D is o.k.
Also I have never tried to catch the moment in photography, for me is much more important to concentrate and and make "atmosphere" in which I can direct the "Decisive moment" and for that kind of approach also 4x5 is fast enough.
This is just my view and my feelings with both systems hope it helps.
Best,
Tomo