The only experience I have is in F-Class, and I don't think any of the top competitors will use a 6mm for Long Range. Almost everyone is going to 7mm's so that they can shoot the 180 gr. bullets, more recoil but the wind bucking capabilities of the 180's are worth the beating.
The US F-Class Open team used the 6.5-284, now they are going to 7mm, possibly the 284 Win. or 284 Shehane. I guess what I'm saying is that the 6mm's might be more accurate but in F-Class, if you make a bad wind call the 6mm will get blown into the 8 ring while a 7mm will get blown into the 9 ring, and one point is one point at the end of the day!
-X3M