I would tell you to go with a regular 6BR. The 6BR might not be as fast as the 6XC or 6x6.5x47, but it will still do the job out to 1000 yards. You will just have to put a little more MOA into the scope. I have owned both a 6x284 and 6MM AI, and after I shot a 6BR, I wondered why I even bothered with 6x284 or 6MM AI. Don't get me wrong, the 6x284 and 6MM AI are awesome cartridges but for varmint hunting I realized the 6BR was perfect. Plus, the 6BR is easy to load, is super accurate, has excellent brass, cheap to load, excellent barrel life and it's easy to find components like dies and brass. Currently it's pretty hard to find dies and brass for the 6x6.5x47. Plus the brass is really expensive.
Also, ask yourself how far will shoot on an average bases during varmint hunting. For me, I shoot varmint on average from 100 to 600 yards with the occasional shots past 600 out to 1000 yards. If you think your going to average 600 to 1000 yard shots than I would tell you to go with either 6x6.5x47 or better yet go with the 6x284. But if you think most of your varmint shooting will be done from 100 to 600 yards, go with the 6BR. It will also depend on what type of varmints your shooting. I shoot ground squirrels and Pdogs, so I don't need to much power to kill those small vermin, even at the longer ranges. But if your going to be shooting groundhogs or woodchucks, at long range, than the larger 6mm cartridges like the 6x6.5x47 would be the better choice.
When I Begin having varmint rifles, I was told by many to have a 6BR built for varmint hunting. However, I wanted the 6x284 and the 6MM AI because the speed factor. Once I realized how much powder I was burning, how much money I was spending and watching my buddies with 6BR's out shoot me I knew I needed to have a 6BR built.
I will say this, from what I have read and heard about the 6x6.5x47, it sounds like a great cartridge. Also, if your going to build a repeater, well than the 6x6.5x47 would be a better choice than the 6BR. But if your going to build a single shot, go with the 6BR. Trust me you will super happy with it.