Given only the two choices, I'd go with AR15. Ready parts, easy to maintain, somewhat less "tricky." My personal choice, not in the ones ,is an AR10. Similar familiarity with the 15. The reason I'd go for a 7.62x51 in a SHTF scenario is to be able to carry only one rifle. I would assume either having to move around or, if in a fixed position a limited ability to acquire additional equipment. One to use and maintain. So, I'd have the semi auto capability to deal with several people, as well as the longer range to deal with someone trying to keep me pinned down, or someone with some barricading, cover or armor. While the AR15 would deal handily with the semi part, I would be at a disadvantage if a group were clever - one standing off to keep me pinned down while perhaps only one or two others move in. Instead of having to put one rifle away and get out another, I'd have just one to use and maintain. With a 7.62 I would be able to reach him. Likewise, some light armor or barricading would render the AR15 ineffective. The 7.62 could punch through more. I realize this also would be an argument in favor of the .30-06 of the Garand, but I feel that is outweighed by the fiddling needed. Also, my eyes aren't what they used to be, so it is helpful to be able to attach optics, even if as simple as an Aimpoint.