Howdy Mr. Deadeye,
Both will do a good job, but both will need triggers and tweeking and some quality handloading. I would go with the one with the fastest twist! FYI, get on a search engine and search for"1000 yard benchrest" Go to "Page" NW Benchrest" There you can go to the results of their last shoot{April 12th and 13th} and they list equipment used. The 6mm is doing very, very good at 1000 yds, so 600 to 800 should be some what of a breeze. Good luck, and keep shooting center!!!