The seem very comparable. The only additional feature that I believe Faststone has is the "draw board" where you can annotate, draw lines, circles, squares, etc. Also the screen capture is very powerful... window, window object, full screen, rectangular, or arbitrary polygon.
I tend to use the screen capture - crop - annotate - save workflow when making tutorials pretty much completely within faststone... so it works for me
It is also (as is vnview) free for non-commercial use.