LOL...well...I dunno...18.3333%?

But seriously, Ultegra stuff uses a little better materials, a little lighter materials (generally) and better finishing than it's little brother 105. If your comparing year for year. As far as I know, when buying replacement pads, the packaging states "Dura-Ace/Ultegra/105", so for aluminum rims there is no difference. So to state "how much better is it" goes down to would you like to pay a little more for some additional stiffness, a little less weight and a little more beauty.

If your comparing 5700 105 to 6700 Ultegra, not sure your going to notice a huge difference in actual performance, or it will be marginal at best.....at first. New 105 stuff is extremely functional with but uses a bit more steel vs. aluminum here and there...But personally, I don't think it's nearly as good looking as the Ultegra and DA gear, and I don't believe it has nearly the longevity.