That depends on what you need it for. If you need lighter and thinner then go with the K56CM. If you want a little more kick and weight/size doesn't bother you then you'd get slightly more performance out of the K55VM.
I've got a powerful gaming laptop I can tote around everywhere but my daily workhorse is my ultrabook... It's light enough to carry around for hours under one arm.
This one will be my only computer, so it should be okay to carry around. I have min. 1 hour train transport every day each way. So it would be nice if it was light and thin to carry, but I still want to be able to play games in decent graphics.