
Indeed.
The extra-sensitive center point on the 6D is a gimmick more than anything and has no real advantage under real life shooting over other similar models.
5D3 can focus in very dark environment already, anything darker is almost pitch black meaning the resulting photo will be unusable anyway like you described.
The 5D3 AF is better than 6D, no argument with cheaper price.
For me 6D is better for me than 5D3, lighter and smaller body(because I travel and hiking a lot), built in wifi(very usefull "gimmick"), GPS(good for travel), exchangeable focus screen.