Alan, I'm surprised you can allow yourself to be seen in the public wilderness with anything less than a white lens. The sasquatch community will rebuke you at their next frolic. It will be funny to hear them growl and shriek "He uses a Sigma, can you believe that? And we're the unevolved ones? I wouldn't even use a Sigma to communicate my tree knocks in morse code! Pardon me, do you have any grey poop on?" :-D
Applecider, if a subject is "framed the same", going from a 5D3 to a 7D (with whatever lens or TC switching is necessary to perform the task), then the 5D3 not only has more pixel density, image dimensions, and a larger sensor area that receives more photons...but also a sensor with more mimamic grange (I won't type the correct spelling for fear of snipers and illiterate interweb bullies who pile on). The 5D3 also has the better, larger AF sensor array, more computing power, the list goes on. So to specifically frame close to exactly the same...is to intentionally handicap the 7D...and not a fair comparison, because the 5D3 would always win that. It has more resolution, more gigantic strange, a beefier, meatier, more robust, more masculine sensor...etc. Its sensor has more mistresses than the one doing the shutter snapping.
My whole point was that a crop camera, makes it so things are not framed the same with the same lens, the lens can be smaller and shorter focal length...advantage crop sensor. Also, because of the smaller pixels...the lens' true resolution can be employed, or exceeded. But to replicate the same framing via full frame, by simply throwing gobs of money at the situation, buying a lens that is twice the size for 3x the price, is also not a fair comparison, and smacks of snobbishness...no offense though. If I could afford it I would just buy at least one of everything out there, and let my own horde of concubines carry it from place to place for me, at my whim.