The 8 speed offers a larger range of returned value. Coupled with a 3:21 axle ratio, the 8 speed offers lower off the line power than the 6 speed with 3:92s. In the higher gears, you will get improve economy with lower RPMs at highway speeds.
You have the option of manually selecting a gear to optimise the towing power needed.
I tow a 7,500-pound trailer/tractor combo around town with my 2013 8 speed with 3:21 axles, never felt the need for more power. The 8 speed does a nice job of keeping the engine it is power band.