The 6's and 6.5's are good for low recoil. When you go to the 7's you are pushing 50 more grains of lead but you can get much better BC.

The 6 Dasher recently set a 1000 yd world record. I have seen several clean all three relays at 600 and very close to that at 1000. The 6BR, 6XC (limited brass from Norma) have done well also. I am interested to see if anyone jumps on the 6 SLR.

Formulating a plan may include a trip to the next 1000 match and see what the best shoots. ask around. I believe that local altitude and range(wind) conditions may have some influence on your decision.