First, if you haven't read enough posts here to know this, I'm openly biased towards Phase One backs – I work for a P1 dealer.
The big thing that jumps out of your post is focus. I can't tell you if you'll find the focus on any medium format system satisfactory or not. Then again, neither can anyone else. You're absolutely, positively going to have to do a real shoot (or at least one that comes as close to "real" as practical) with a medium format system to find that out. The DF+ and H4D/H4X both have very good autofocus systems that are greatly improved on their earlier brethren (from that point of view you need to weight the opinion of responses about the H1/H2/AF or DF (non plus) bodies), but fast moving kids are definitely challenging subjects for AF-only shooting. I think it will depend a lot on your preference for how wide open you wish to shoot and how much lighting you have in your studio environment. If you have a brightly lit studio and don't need to shoot wide-open (at least for the fast moving kids) then I think the AF on either system is good enough to warrant your own testing/evaluation. You're never going to track a child running around the set wide open with any meaningful hit rate with any medium format system like you could with a Canon and a fast prime and servo tracking. But to quickly move your body to compose for a shot, autofocus, and snap off a few frames in a few seconds... yes you can do that (in my experience).
The rest of the issues are much easier; here are some responses I don't think anyone on this board will disagree with:
- yes it will print large images like 30x40 with better quality than a 5D as far as tonality, detail, texture go (I can send you some of my Credo 40 raw files to print as your own practical testing if you'd like)
- dynamic range is an improvement over your 5D3 (though in-studio dynamic range isn't a huge deal as you should be able to simply add more fill light as needed in most scenarios)
I'd also argue the following more subjective points:
- You'll love skin color from the Credo compared to your 5D3. It will look better and require less post processing tweaking to get there.
- You'll love the look of the Schneider LS lenses; they are sharp even wide open, and fall off nicely
- You'll like using the larger viewfinder of the DF+ compared to your Canon, especially with your damaged eye
- You'll love, for the limited amount of location/environmental portraiture I see on your website the ability to sync at any speed (up to 1/1600th with the Schneider leaf shutter lenses).
Things you didn't ask about but I think are very important:
- tethering via USB3 or FW800 is very fast, so if you already shoot tethered or want to reserve that possibility you'll be quite happy; the cable is deeply recessed and doesn't pop out accidentally very easily
- You'll enjoy having the camera you want/lust-after for the work that you do rather than settling for something else. This can have a non-logical-but-nevertheless-true-because-we-aren't-creatures-of-pure-logic impact on your shooting style and confidence. I don't know for sure that clients notice or care what camera you use - I get the argument and I think it does apply some of the time with some clients. But I am quite confident you will notice. It's nice to work with tools you love (which is why it's even more important you get to do a real test with the gear to make sure it is a tool you love).
So I'd get your hands on one and see what it does for you. If you love the final result, find the autofocus is not going to meaningfully hold you back, and you enjoy the shooting process - do it. If not, do something else (or do nothing).
