#5.18 10 speed vs 11s cassette. Which one is REALLY better? Shimano 105 Ultegra Dura-Ace

http://tidd.ly/d37674bf Secret clearance page at Merlin Cycles
I do make an upgrade on my CAAD10 bike from 11 speed 105 5800 components to 10 speed Ultegra 6600. What made me want to make this change???