Not sure if anyone else has said it, but the "reach" of the 7d vs a 5d3 is BS. Crop into the 5d3 frame and it will still be sharper than the 7d. I did lots of testing myself to confirm this. The only time I touch my 7d is as a backup. Go for that glorious 5d3 and you will love it.
based on your suggestion, i wonder:
1. why buy telephoto while i can crop, and
2. should i sell my 5d iii to buy nikon d800 since its sharpness will maintains much better than 5d iii after cropping due to its resolution...
yes, i still have my 7d (will keep it as back up) and 30d (will keep it for experiment purposes). everyone has his/her own philosophy about utilizing his/her camera right?
Huh? I assume you are being sarcastic? Longer lenses are always better than cropping. Unless, of course, the lens is just really really bad.
For any given lens, a 5d3 gives such a better image than a 7d that even with it cropped down to the same field of view as the 7d, then unscaled to 18mp, it can look just as good, if not better than the 7d.
And no, I don't think the d800 would be nearly as good. Canon lenses are awesome.