The Speed is not the interesting thing the difference in Power Consumption is
i made a small test on that though these are in the wild shots not real scientific but i came to the following though this is also based on Nvidias old Shader Encoder not the new Kepler one and i came to the following result @ comparable quality on a 4 Core Intel SB system.
all Decoding CPU all overall system Power Consumption (out of the wall measurement) all using CABAC High Level 3.1
Quicksync (SB I5-2400 not S) = 110W
x264 (with some preset optimizations) = 130W (though im confident it could be lowered and would stilll look as good with some deeper source changes)
Nvidia 460 GTX (full/partial encoding) = 180W (full and partial encoding differ in the final visual result)
all reaching same Speed of around AVG 180 fps @ 720p 30 fps 3 mbit
though obviously more interesting it becomes the Higher the Bitrate gets and CABAC drives on