Like Ackleyman said, the 60 grain Sierra bullet is awesome, both for accuracy and performance. I have been shooting the 6 x 45 with the AR and bolt-guns for about 16 years, and have built piles of AR uppers over the years. The 12 twist does a great job as far as fine accuracy on the 60 and 70 grain bullets, and for the folks that plan on shooting the light bullets, I have always suggested the 12 twist.
I have had both the 12 and 10 twist and have never actually been able to say that I got better accuracy with the 12 twist and the light bullets over the 10 twist. Since I hunt white tail with my 6 x 45 and use the 85 grain Nosler partition, I have gone strictly to the 10 twist on my own rifles.