I've posted this before, but here is the differences in crops in regards to formats.
Keep in mind a P30, is not a 1.3 crop it is 1.24 where a almost FF mfd P45 is 1.14, so the differences are not near as pronounced as the sales literature suggests.
I know that this forum loves to post mtf graphs and micro sharpness, but those are just relevant when your bleeding your eyes on a computer screen.
To me the one reason to shoot medium format is the look of non aa filtered CCD's. They just look different than cmos and my studio and I have worked thousands of images from most makers of cmos cameras (Nikon, Canon, Panasonic, RED,) and CCD non AA filtered cameras (leica, Phase, Leaf, Hasselblad, Kodak and Fuji) and there is just a different look to the ccd cameras.
I won't say 3dish, or any of that and I'm not comparing micro detail but I believe the ability to work a file deep with MFD is easier than a cmos file and produces an overall different look than a dslr.
As far as cameras I'd use what camera you feel comfortable with, but I would never buy without testing in some form, because everybody has a different perspective on cameras, files, lenses.
I'd test whether it was a $2,500 dslr or a $30,000 medium format system. It's not just the costs, it's the years of living with it that matters.
I'd also test and shoot something relevant. No alleys, brick walls, or leafs in the backyard, but load up, go out and shoot something pretty. Then you can work the files in post and find the system that's right for you.
You will hear a huge volume of buzz around the Nikon D800 and I'm sure it's a capable camera. You will also here a lot of people say it's not the camera, it's the artist.
Both are true, but the camera really does matter and it always has. Nobody spends more time with their eye on a viewfinder than a movie first operator or DP and though they rarely own cameras, usually rent, they have their favorites and will go to almost any length to use them.
It's not unheard of to hear a dp absorb part or all of a cinema camera and lens package costs into their fees, just too shoot with the camera they are comfortable with.
It's the same for stills and the one plus for medium format is (at least for me) is I continue on with the same equipment for a long time. My dslrs, maybe 2 or 3 years, my mfd backs, easily 3 times that.
As far as reliability, my Phase backs have never had a repair and I've worked them damn hard. My Contax(s) just one repair where some assistant put a thumb through a shutter curtain.
My Canons and Nikons have all gone in for shutter failure, or focus adjustments.
For weight, I don't look at the camera, I look at the case and the difference between my Canon and Contax/Phase case is less than a pound.
Still, the only answer is test and decide for yourself.