Reach? Assuming we all care about IQ, it's a consolidated opinion by now that a cropped image taken with the 5DII looks better than one taken with the 7D. Better still, it's more than an opinion, it's a fact.
It's not only not a fact, it's a fact that that fact is flat out wrong.
Well... you're right. First because I wrote 5DII instead of 5DIII, which is the cam I was actually talking about, second because I missed to specify "at high ISOs", which is often the situation you're into when shooting fast action sports, birds or wildlife in general. I remember several threads, and some comparison sample pictures also, which appeared here on CR (and massively in some other forums) concerning this matter: not considering the loss of resolution, simply IQ-wise, hi-ISO FF crops looked better than uncropped APS-Cs to my eye. But, please, don't ask for links, I'm too lazy to look for old CR threads now, let alone other websites... I just want to mention what I always say, that a 8 MP clean image is better than a 18 MP noisy one. You can always apply NR, but then you lose detail, anyway.
7D still does better when reach limited even at high ISO than the 5D2, it simply has a more efficient sensor per area
the 5D3 sensor is a little bit more efficient per area than the 7D so you do get a bit better SNR from the 5D3 in all cases but the extra density of the 7D still makes it better than the 5D3 for reach limited scenarios in most cases, even at ISO3200 I have some real world bird pics I took with both and I prefer the 7D versions
if you scaled the 7D reach limited subjects down to 5D3 size the SNR isn't THAT much different, although a bit worse, but they have less moire/de-bayer artifacts; if you keep them full res they show a lot more detail (for high contrast subjects not in the darkest parts of the frame even at ISO6400 they pull a lot more detail)
less contrasty, darker subjects in darker parts of the frame might come out worse overall at high iso with teh 7D than with the 5D3 though even when reach limited (when not reach limited 5D3 obviously better)