Many medium-format backs can be adapted to a wide range of medium-format cameras—Mamiya, Phase One, Contax, Hasselblad V and H, and even view/technical cameras. Thus, when an exciting new sensor (or camera body) becomes available, you need only upgrade that. With a DSLR, the sensor and body are a unit—if you want to upgrade one, you have to upgrade both (this is also the case with the Leica S/S2 and Pentax 645D cameras; they have built-in sensors, not separate backs).
Most medium-format cameras accept a number of leaf-shutter lenses. Leaf shutters offer the advantages of quiet operation with less "recoil," as well as flash sync at all shutter speeds, even at full flash power (focal-plane-shutter DSLRs have maximum flash-sync shutter speeds of 1⁄300 sec. or slower, and those that offer "high-speed sync" do so at much reduced flash power).
Many newer medium-format digital cameras provide touch-screen operation (only a few DSLRs do, none of them high-end models) for quicker and easier shooting. This, plus versatile tethered operation, and the superb image quality combine to make medium-format digital the ultimate choice for many pro and fine-art photographers.
Doing The Math
The more pixels an image contains, the larger you can blow it up before the pixels themselves become visible to the eye. Of course, factors like viewing distance come into play—the farther you are from a print when you view it, the less likely you are to see the individual pixels making up the image. Viewed at close range, those giant billboard images aren't very sharp—and they're pretty pixelated.
For the print publishing industry (magazines like this one), you generally want 300 pixels per inch. Thus, simply dividing the image's resolution in pixels by 300 tells you how big you can publish it in inches: an 80-megapixel image from a medium-format camera measures 10328x7760 pixels, meaning you can run it 34.4x25.9 inches at 300 ppi. For a 36.3-megapixel image from a Nikon D800, pixel measurements are 7360x4912 so, dividing those figures by 300, you can run it 24.5x16.4 inches at 300 ppi. Images from 24-megapixel DSLRs measure 6000x4000 pixels, so they can be published at 20x13.3 inches at 300 ppi.
These are measurements of area, of course. Resolution is a linear measurement, so many lines per millimeter or per picture height. An 80-megapixel image contains 2.2X as many pixels as a 36.3-megapixel image, but, all other things being equal, it provides only a 1.4X increase in linear resolution. That's still a goodly increase, but not the more than double you may expect from the more-than-double pixel count. Similarly, going from a 24.3-megapixel DSLR (6000x4000-pixel image) to a 36.3-megapixel DSLR (7360x4912-pixel image) produces just a 1.22X increase in linear resolution (lines per mm or per picture height). But in each case, file sizes increase with the total number of pixels, not the linear resolution: Double the pixel count, and you double the file size—and the space required to store the image and the processing power needed to process it. Note that amount and type of compression (none, lossless, lossy) also will affect file size.