30 cal..all the way
That's a valid point as far as most stores go. However, online I think 270 has great availability. Further, I have seen 270 available long after 308 has sold out during panic times. There is not a problem with availability for 270. That said, I encouraged 308 based on the OP's criteria. If I based ammo availability on a few gun stores, I'd have no idea what availability really looks like.I personally prefer 308 with heavy bullets for hunting. Plus I see a TON more ammo for 308 on the shelves than I do for 270. Just for ammo availability alone, I'd pick 308.
For perspective, I'm in different stores in different cities looking at ammo 2-4 times a week. (I travel a lot for work.) I see 1-2, if any, different brands/weights of 270 in most stores. Whereas for 308, I see several loads in several brands in every store. Prices are still pretty terrible for most of it though.