I moved from a D300 to a ZD on a ADF-II then o a P30+
All I really shoot with it is fine art figure in the studio, either strobe or natural light.
As Guy has said, the crop factor matters a lot! In the small community here, I have just bought his 35mm lens ( third hand!), and I'm still lusting after the 28mm.
I now have finally got my tethering working, and if I had actual clients, rather than just doing this as a hobby, the wow factor of C1 coupled with iPads would be a heavy factor in leaning towards P1. Look closely at the software what ever choice you make. LightRoom does not work well with P1 files.
Another factor for me is that once the MF vs FF dSLR bridge is crossed, the lenses available become a factor, I'm still using the pre digital lenses, and they are VERY good relative to the Nikon primes. The D lenses while better than the old ones, for me, are some what marginal diminishing returns. For my work, a leaf shutter is not really that important.
I cannot fully explain this effect, but somehow, I think that my rate of growth in what I want and can get from an image has been much faster with MF .... some how, I just could not get there ( where ever that is ) with the Nikon .... I tried a D3X, and it was just more of the same. The discussion about texture has something to do with it, but there is still more in it than just that. Vague as it is, I'm really happy with the P30+
Do drop me a note if you would care to see some of my work.
YMMV
Dave