THIS.
5.56 (aka .223*) is the quintessential round for the US military, and thus the militia at large (that means you).
7.62x51 (aka .308*) is the next-most-important US military round, preferred for - ahem - long range applications. It is also the most universally applicable round, suitable for general purpose use a la Jeff Cooper's "scout rifle" concept.
Follow our military's lead, both for long-term availability and proven usefulness.
* - The .223 and .308 versions are very slightly different from their gutsier military 5.56 and 7.62 incarnations. Get the military version wherever possible, especially when picking a barrel (the "civilian" version may not withstand the "military" ammo).