Get a AR-15 A2 Style, the flattops are nice until your optic gets destroyed, plus you CAN add rails to the carry handle of the A2 anyway. a win-win IMO
but deff. go with the 556 AR, If 30. 06 was still cheap and common I'd say get the Garand, but in the end the AR will be more satisfying. its much more versatile.
barrel length depends on your environment, if you plan to hunt get a 20 inch barrel, if its SHTF/home defense, I'd personally go with a shorter barrel I.E. 16 inch.
55 gr. works magically with the 20inch, it was reported to take off limbs in Vietnam, but for the carbine lengths id go with heavier round, 66. gr and up.
THey tend to use heavier rounds in Iraq and Afghanistan, but they get mixed reviews