Less shoulder gap between new case shoulder and chamber
More brass availability (considering you can use 30-06 brass)
More efficient amount of powder to velocity ratio
Longer barrel life
Less muzzle blast
Easier to find an appropriate powder
I have had a 264 win mag and now have a 6.5 rem mag. IME it is much easier to find a load pushing a 140 gr bullet at 3100 fps than trying to find one in the 264 win mag pushing the same bullet at 3250 fps (my spreadsheet shows red highliter all over it showing pressure signs on the 264 win mag page). It is hard to find a slow enough powder to use in the 264 win mag. With the 6.5-06, 6.5x284 or the 6.5 rem mag the case capacity is less and many powders like IMR4350, IMR4831, RL19, H4831, RL22, IMR7828, H1000 or RL25 or entirely appropriate.
Personally, I like a bullet that starts out between 2900 fps to 3100 fps because if you have a close shot the bullet may not get squirrely on you and blow up or pencil through. I'm a heavy bullet 3000 fps guy and the 264 win mag is too much. The 6.5-06 would be just right.
Edited by sakomato - April/02/2009 at 20:32