... Then there is the Nissin 866 with a built in sub flash for fill in, a feature I think would be useful. I think both of these have high speed sync which you might want if using for outdoor fill. Price points are roughly 1/4 and 1/2 that of the 600 respectively....
On the flip side you tend to get what you pay for with Canon stuff, and it's really a matter of whether the "shortcuts" of the third party stuff will cause you problems worth spending for Canon...
Some time ago I bought my first external flash... was toying up between Canon's 580EX II and the Nissin 866mkII. I bought the Nissin (new) and am very happy with it not just because it was less than half the price of a new Canon 580EX II, but because it works very well on my 7D. Note that the original Nissin 866 (not mark II) does not have full compatibility with some of the later features.
The sub flash on the Nissin 866mkII really works well, and when I use it as a remote flash, and also use my Canon 7D's pop up flash to fire too, I effectively have a 'mini 3 flash' system. That is:
1) using the Nissin866mkII's main (powerful) flash to bounce off the ceiling, or off a wall
2) using the Nissin866mkII's subflash for some slight fill side light and
3) using my Canon 7D's built in flash for some direct fill light.
Thankfully I can control each of the 3 flash's output separately, that's cool. Creating some quite interesting possibilities. I realise that the 5DmkIII doesn't have a built in flash (which I would like Canon to change!) - but still, I'm just sharing with you my experience, and being v happy with the Nissin. The AF assist works very well on the 7D (eg dark room, it will focus on a blank surface).
If money really is no object, I'd probably go with the Canon 600 EX-RT - for the radio technology, which is certainly handy. Otherwise - I can recommend the Nissin - which I've learned to use - and it does very well with ETTL-II being utilised. Hope you go well with your decision.