As the number of pixels on sensors continue to increase, many photographers shooting DSLRs like Canons and Nikons believe that either the 35mm camera body sensors will approach the results of medium format sensors, or that a physical limit to pixel numbers will be reached. While the latter may be true some day, it is probable that the size of the sensors will continue to be different.
While a "full frame" Canon or Nikon sensor can be fitted today with 21 or 24 megapixels, Hasselblad, Rollei, Mamiya and other camera systems and backs are becoming available with up to 60 megapixel sensors. Hasselblad also reportedly has plans on the drawing board for 150 megapixel sensors. The medium format sensors will always have more potential because they are three to four times the size of 35mm size sensors. As long as this is true, the larger sensors will be able to accommodate the capture of greater tonal ranges and more detail. To make it possible for Canon, Nikon, Sony, Pentax and the others to achieve this, they would have to fundamentally change the size of their cameras and accommodate larger sensors, or radically alter the technology. But the latter approach might also be duplicated by medium format manufacturers.
DSLR manufacturers are also still promoting the APS size sensor cameras, which actually constitute the bulk of their DSLR sales. These sensors have already reached an amazing 15 megapixels, and now 18 MP with the new Canon EOS 7D, which may be a practical limit for the sensor size. In fact, manufacturers are now looking at reducing pixels, instead improving pixel sensitivity to increase tonal range and sharpness. After all is said and done, something between 10 and 15 megapixels may ultimately become the ideal level for APS size sensors.
From a professional's standpoint today, where is the sensor size advantage? It completely depends on the type of photography you do, and what your competitors offer. On the high end, commercial and fashion photographers using medium format digital cameras generally book much higher paying assignments than those using the EOS 1Ds or Nikon D3x. Their clients can see the difference and are willing to pay for it. Earnings have to make a difference when deciding between spending $8,000 on a camera, or $27,000. The business and the markets are there, however.
Those in the portrait business have to consider the size of the wall portraits that they sell to most of their clients. If they market and sell 30x40" and larger wall prints, there will also be a noticeable difference between prints from a 50MP Hasselblad, a 24 MP Nikon D3x, and a 15 MP Canon 50D. The sensor size will make a visible difference in how spectacular the images look. Many get spectacular prints from APS size sensor cameras, but when comparing 30x40" prints, the difference is evident to educated, high end clients.
The wedding market is quite different, with it's typical albums being in a 10" or 12" size, and wall portraits typically maxing out at 16x20". Photographers with old Canon 20Ds and Nikon D70s can continue to produce excellent quality results, without placing extensive wear and tear on their D3's or 5D's. But let's not kid ourselves. There is a difference in images from full frame and APS sensors. You yourself can see it on your computer monitor and on the print.
During decades of film photography, there was no getting around the fact that a larger piece of film produced a photograph with greater detail. There was no comparison between a 35mm landscape blown up to 16x20" and a 40x60" wall print photographed with an 8x10" field camera. Even the difference between a 4x5" and a 6x6cm negative was easy to spot. Today's digital situation is no different. We each have our markets and we continue to use the most appropriate equipment for the assignment.