I would jump at that H3dII.
Disclaimer: I've never used an H3DII50.
I like Phase Backs, don't get me wrong. But H1 versus H3, well, I had horrid problems with the H1 back in the day. Horrid, miserable problems. I know that firmware has come a long way, but the H3 is just put together much better.
The P45, to me, was always too sharp for people, if that makes sense. I always thought its color rendering for people looked too much like E100 Kodak slide film, not a bad thing is most cases, but i never reall liked it. It is a hell of a back.
As to ISO, I don't know this for a fact, but I would assume that the newer H3dII50 has better ISO performance than the p45. I used the plus version and 200 was pushing it a bit. The H3d39 had nice noise performance up to 400, and 800 needed some work but was usable for the right shot. I would suspect, but don't know, that the 50 would be better.