Q: Should I save my scene file as .ma or .mb and what's the difference?
A: .ma stands for Maya ASCII and .mb stands for Maya Binary. Ma files are essentially just text files that describe all the elements in your scene. You can open one up and take a look in a text editor (and change things too). Mb files are only machine readable, don't bother opening it up in a text editor.
Ma files are larger sizes than mb files containing the same scene info, so if disk space is an issue, save as mb files. However, if your scene gets corrupted, sometimes you can locate and solve the problem using a text editor if it is an ma file, whereas an mb file will be just junk. So, if you're worried about the scene being corrupted, maybe save it as an ma. Finally, you can edit things like versions of maya in ma files, like post #10 above.
Ultimately both will work. Some people opt to keep the latest version of a scene as an ma, and save older versions as mb files to save space.