Well, hopefully one of the ammo "gurus" around here will correct me if I'm wrong, but I would think the SA will stay "fresh" longer since it hasn't been exposed to air since it was battlepacked. Although it seems like the main concern is moisture :)
I could kick myself for not buying more ammo when I had great opportunities... a few years ago I got a tub of the SA stuff (denel marked, 2880rds in 30rd boxes, all sealed in a big plastic bag inside the tub, not the battlepacked stuff) for *$200*, and I could have gotten ten more for that price. Oh well, you snooze you lose I guess... the same place (actually an Eckerd's drug store in northern NY) also had beautiful portugese 7.62x51 by the wooden case, $2 a box ($100/1k), one time, but I only bought 300rds because I didn't have any 7.62nato rifles at the time :P
I do wish the mail order ammo companies would offer more decent quantity discounts to people who want to buy 10-20k at a time. I hate to think what they must be getting this stuff for!