https://www.dpreview.com/forums/thread/4020014"I attended a Canon event a month ago, where I got to play with the 600EX II-RT, and talk to Canon (sales)staff
The main difference is the recharge time, where the 600EX II-RT with the new external battery pack can support up to 14 shots per second - I guess this update is mainly for those using the 1DX II (it is an Olympic year after all)
Other differences are the included filter/diffuser system which has a different design and the menu system has been redesigned - based on the time I had with the unit, it was a little easier to navigate, but not enough to sway me one way or the other (btw. the ST-E3 unit is still the old one, and I do like the fact that menu on the ST-E3 and 600EX-RT are identical)
For my use (mainly portraits with a 5D3), I see no reason for chosing 600EX II-RT over the original"