I think the most primary issue would be that if you fly a second OW airline frequently, then that would definately be the one to look at. As we all know, an airline treats its elites better than someone else in the alliance.

With my travel plans, I'd be hard pressed to consider anyone else. I just don't cover non-AA areas frequently enough for it to matter, so I'd stick with AA. Personally, I really like South America, so I'd probably look into the LA/LP program and take advantage of the 40K km (24K miles) w/2 J long-haul r/t and use some of their business class discounted flights to meet that. Their Sapphire program also gives you upgrade coupons that are valid for any flight (with fare restrictions), so you can upgrade that US-SA flight to their Premium Business. Then again, that only makes sense if you want to fly them.