I will be building an AR15 (lower on order) for deer hunting and long range target practice. Do not plan on hunting over 300 yards, but would like to be able to target practice longer distances. I have started reloading and plan on someday reloading all my ammo, but would also like to be able to find it if I need to buy it. Read some information on 6.5 Grendel and 6.8 SPC, but some have been saying the 6.8 is better?

I plan on buying an upper to use on the lower I will build and would like to know what you would use for an upper?