There has always been debate about the BEST format. In the case of film it was between 35mm, 120 roll film and sheet film with just about every size imaginable from 18 x 24mm (smaller if you want to include sub-miniature) to 20″ x 24″ and beyond.
With digital something the same has emerged with the size of the sensor becoming a matter of real importance to a lot of photographers. But does it really matter?
The answer probably depends upon what the images are to be used for, and whether or not you are trying for the ultimate quality in massive enlargements.
If the images are intended for the web almost anything will do. Certainly any camera currently on sale. Ditto for 4″ x 6″ prints for the family album.
Above that things are not so clear cut. I have seen some wonderful A4 prints produced from 1.3MP cameras. Six megapixel camera are capable of great 60 cm x 100 cm prints. Equally I have seen lousy prints from both 1.3 and 6MP, as I have seen lousy prints from Leaf and Sinar large format sensors.
The photographer and his or her craftsmanship still remains the crucial factor. A careful worker who uses a tripod, considers depth of field, ensures accurate focus will always produce a technically better photograph than someone who quickly snaps a shot of the same scene with the same camera. So it follows that all other factors being equal the final print will be sharper. This has nothing to do with aesthetics, or the size of the chip, just technique.
If the same photograph was taken with, say, a Leica M9 fitted with a Summicron and a Nikon D3S with a Nikkor both on a tripod at ISO 200 (for example) enlarged to the same degree I would suggest that there would be little apparent difference in the final result. Pixel peepers MAY be able to tell which was taken with what camera, but that is not the real world.
Where there will to be a difference is when the cameras are a full frame versus a 2/3 chip. But even then it may not be readily apparent if the print is post card sized, and even today that is the most common size.
I don’t print larger than A3 all that often and for most subjects I think that I’d be happy with a FF, 4/3 or APS-C. Having a Full Frame would give me my traditional focal lengths back, but having used an APS-C for some years I’ve gotten used to fitting a 50mm when I’d like a 75mm. I’m limited to the equivalent of a 24mm on the wide side, but that’s alright as I’d never owned anything wider than a 28mm until I went digital.
What is most important to me is a decent buffer size, a 100% viewfinder that is usable in all lighting conditions, the ability to knock out good A3 prints and a camera I can operate without having to think about the controls. The size of the sensor is almost incidental.