Neither application presents much of an autofocus challenge unless conditions are very dark. The 6D will give you similar image quality to your 5D III in a smaller, less expensive package. It's autofocus is similar to the autofocus of the 5D II. That camera continues to be used, very successfully, by a large number of professional photographers. For events and weddings the 6D's only drawback is its 1/180 flash sync speed.
The 6d is one of the cameras I haven't used to be honest... To give a bit more context, over the years I've used (not necessarily owned):
- 5d
- 5d2
- 5d3
- 1d2n
- 1d4
- 50d
- d800
- d700
- d3s
I realise weddings and events aren't the most demanding in terms of AF, and 9 times out of 10 the focus and recompose technique isn't an issue, but when you've tasted the good stuff
It's definitely interesting though that most people are recommending another FF body, despite the AF disadvantages encountered in my budget. Maybe the IQ and lack of full-frame of the 50d is having an impact on me more than I realise... I like how people on this forum make you think - always so helpful 


