There is no difference!
It's ultimately about what YOU feel comfortable working with and what you are used to.
So, anyone have ever proposed a way of testing this, and conducted such tests? (or can?)
You can run a usability experiment where you give a set of test persons the same task, for example: "Import 20 RAW photos from a camera, edit them, burn them to a CD and make a slideshow" (of course, way more detailed, but you get the hang of it).
This experiment would then measure how long it takes for the task to be completed, and how satisfied the test persons were.
The problem is that all those test persons should not be biased towards either platform and should have no experience with either one. Where do you find such people? It's hard to imagine you would find someone who hasn't used either. And if you want to find people with experience in both, how do you define "equally experienced"?
Then again, the only difference (so to speak, "variable") in your experiment should be the platform (i.e. Mac or Windows-based PC), so that's another problem. You would have to run the test with exactly the same software and hardware to be able to say: "Mac is better than PC, given the same software and the same task, and the same people". If the software changed, you would be bound to say: "Mac is better than PC, but we used different software, so, it's not a good comparison". Same for hardware.
You could obviously also test professionals in the same manner. And as you can see, even in this very very simplified example, you run into problems that run out of hand when you want to do a real "Is Mac better than PC" test.
Some say mac monitors are better, is this because they are glossy?
Then those people obviously have never seen a good monitor. Glossy monitors tend to accentuate colors and contrast, therefore giving us the impression of a more vivid picture. On the other hand, forget those for editing videos or photos. Their color representation is nowhere near perfect and a professional in video/photo editing would never ever use a glossy monitor.