Maui5150 said:
Reach is another... So the 300 becomes a 480
I still like the 5DIII but some like the crop sensor for length
The 'reach benefit' is mostly an illusion. It's a 'crop factor' not a 'magnification factor'. A 300mm lens doesn't
become 480mm on APS-C, you're just cropping away the outer part of the frame.
A 'reach benefit' for a 7D over a 5DIII only exists if you need more than 8.6 MP for your desired output. If 8.6 MP is sufficient (up to 16x24" / A2 prints), then the 5DIII image cropped to the FoV of the APS-C sensor will give you equivalent IQ at up to around ISO 800 (on the 7D), and progressively better IQ as ISO increases from there (at some point, probably ISO 3200 but certainly ISO 6400, the 7D's noise is so bad that an up-res'd 5DIII image would be better even if you do need all 18 MP).
Some might also cite the 2 extra fps as an advantage, and it is one in theory…but in practice, I believe the superior AF performance of the 5DIII will yield a higher overall keeper rate despite the lower fps.