A system restore point doesn't backup your files, while an image copies everything at the point in time when you created the image.
Images save more than restore points, but they take more room than a restore point does.
A quick read on the Aomei site says that it functions similarly to Acronis - so I'd expect the difference in sizes to be due to differences in how they store and compress the images. As it's proprietary info, I don't expect that there'll be much info available about this difference.
While I haven't tried Aomei, I've been using Acronis since the XP days and don't have any problems with it.
I've tried other imaging software and don't recall issues with it either - but then I haven't needed to use any of them in an emergency for a very, very long time.
