Apple lists all the command options for this:
http://docs.info.apple.com/article.html?artnum=12555
As for user friendliness, all the Apple combinations can be done can be done left handed, with the majority of people using mice with their right-hand this means you can quickly mouse and screen capture at the same time.
On Windows you usually have to use your right-hand to hit print screen (or if you want just a particular window, the finger stretching Alt+PrintScrn). That means mouse, to keyboard, back to mouse, and so on.
Apple also offers more options such as saving to a file (if you do it multiple times, each file is saved with a number at the end) so if you have to make lots of screen shots you don't have to go paste that image somewhere, then go back to where you were, take another screen shot, go back and paste it, etc....
And finally the Mac lets you do a rectangular area, which Windows doesn't (without an add-on package).