This is why I avoid Milan I would stay in the Cathedral for ages
Low end cameras has the main difference in working situations. In optimal circumstances they work very good, but the optimal circumstances are way less wide than better cameras. A month ago or so I had a chance to give a try to EOS 5D mk III. IT was the same as my own camera. But there was no light for mine, and well enough for 5D... This is the major difference Every other differences are minor cosmetic things.
Inside it's less detailed than outside, you'd probably spend years on the roof of the cathedral^^
I see really much differences between my SONY SLT A58 and my old KODAK EASYSHARE M530 Digital. But that's the difference between a Single-lens reflex camera and a simple digital-camera The most important one: With the Sony-camera I can take night-shots, while the Kodak could do that only when there was much light from the streets. So that's mainly the same point that you had with the EOS 5D, the difference between light-situations is really big. But there are also things my small old camera can do better---> for example taking photos of my architecture models, because there are often just small distances between the parts of the model^^
Yes, the main difference is the light, while small cameras often perform better in close range. A friend has a small but rather pricy Olympus MILC and he struggles with the simplest themes that move. For bird photography, he is completely lost, but if we take macros, his Olympus turns into genie mode, and delivers all his wishes. Without any special equipment he takes picture of the left eye of the ladybird.
I discussed about lenses with a photojournalist friend who works for Hungarian News Agency MTI. He told me that the main difference between his EF 400L f/4 and my EF 70-200L f/4 + 2× converter will not shown in ideal light conditions. But his "log" will deliver its best when mine must be packed away in the lack of light.
The main difference between affordable and expensive gear seems to be not in the top picture quality (though usually that differs, too) but in the worst case situations.