If you were actually talking long-range (800-1000 yds) I'd say go with the 6.5 but for mid-range (500-600 yds) I'd tend to lean toward the most conventional cartridge. The .308 with 155 gn bullets can do quite well at the range you plan to shoot, the recoil's not bad and you get to shoot in F-TR class. .223 is also worth considering at that range. Unless the winds are just really squirrelly, .223's ballistics these days are pretty good.