I had a D700 then stepped up to a D3s, and now have the D3s and D4. From shooting with these two "pro" body cameras, I know whenever I pick up someone else's camera that is a more "normal" sized body like a D800 or D7000 (Havn't held a D600, have seen it and assume its somewhere in-between these two)...I can't seem to make my hands feel as "comfortable". I have (very) small hands and yet these smaller cameras still feel lacking, maybe less so with the bulkier D800) but certainly a D800 or D600 with a battery grip WILL not compare to a D3s or D3x body ... I know I was shocked the first time I put my D3s next to my D700 with Battery Grip (when I still had both) and discovered that the D700 with battery grip was larger then the D3s! Anyway, I find the "pro" body style Nikon's to be the height of comfort in a camera.
To mee it certainly seems like you are considering things from the perspective of comfort since IQ is not up for debate as much here. I'm sure you can find some direct comparison files out there, but it sounds like for your uses the minute differences between the two sensors won't really matter, and we know the D3x is a stunner at the low-ISO's.
Personally, I would only consider the D800(or e) as a replacement for the Hasselblad. There are hellish cornucopia of discussions on here about whether the D800 beats MFDB, but it sounds like again from you that the issue here would be the print size, and while the MFDB has larger pixels, I suspect the 36mp of the D800 would yield you the resolution you would need to provide to clients. I only mention this since selling the Hasselblad could free up financial capital to invest in the D3x, and then a D800 if you still needed an "ultra" resolution camera, its nice since you seem to be set on lenses that is is simply the consideration of spreading around your equipment investment and capital in bodies.