For the most part, the differences are weight and features. i.e., DuraAce was the first to get electric shifting, it was the first to get 11 speed. But it all trickles down -- i.e., electric shifting is now available on Ultegra, Ultegra will probably get 11-speed next year, etc.

All of Shimano's product lines can be setup to shift and brake really well. Shifting performance has more to do with how much care was done with the initial setup and the condition of the chain than the product name stamped onto the derailleur. A DuraAce drive train that hasn't been cleaned in a couple thousand miles is going to shift like crap; a Tiagra drivetrain that's nice and clean will shift pretty well.