Guinness and Left Handed are not similar at all. Guinness is a dry stout, with a light body. Left Handed is a sweet stout, with a big, full mouthfeel and body.
For some info about the different styles of stout, take a look here http://www.bjcp.org/2008styles/style13.php
For what its worth, I think Left Handed is a good example of the sweet stout style.