Originally posted by 24X36NOW Actually, there's another; you can use a 1.4X or 1.7X teleconverter to give you the same/similar FOV as an APS-C crop camera,
keep your full frame and full megapixel count, AND you still get to enjoy that nice, big viewfinder.
But you will lose 1 stop on the 1.4x and almost 2 stops on the 1.7x. Plus, you can of course attach teleconverters on APS-C and 4/3's cameras as well.
Judging by your user name I know it's pointless debating this with you, but I'm bored, so I've attached a sample of all that you gain using 21mp FF vs 15mp APS-C vs 12mp 4/3's. Needless to say it's not much.
If I were a nature photographer I'd buy something like the Olympus E-3. You could put their 8" long 90-250mm f/2.8 on there and have a 35mm FOV of 180-500mm! You'd have to buy Sigma's 28" long, $34,000, 200-500mm f/2.8 lens to accomplish that on 35mm.
Now if I were a portrait or landscape photographer then I would go 35mm, but not at today's prices. As you can see in my attached file the resolution difference is minimal between the 5D MkII and the 50D, but the price is not. For the price difference I could sell any old lenses I have an build a wonderful kit for the difference in cost between the two.
When prices come down it may be worth Pentax's time to make a FF camera, but I wouldn't do it if I were them until they can bring one to the market for under $1800.
On a side note, anyone that wants FF for low light purposes might better get one of the 12mp models before they "upgrade" them. DPR now has a review of the D700 up. In it they compare it to the 24 megapixel Sony A900. The 35mm A900 does no better at ISO1600 and up then the APS-C Nikon D300 does.
Edit: Just to clarify because after a second look I realized the image could be misleading. This image is not a crop ratio comparison. It's simply an image resolution comparison. The 5D mkII image I used to create it was originally 5616x3744. Bob Atkins has
a nice diagram of crop ratio's if that's what you want.