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I would go with the *Alliance carriers over CO any day of the week. I would recommend NW, but if you're going to South America, you'd be flying on a lot of CO metal & depending on the fares you'd be paying, you may only be able to earn 50% EQMs in the NW program. That being said, UA & NW are two airlines that have a lot of mileage promotions & the *Alliance has a greater global reach (or so it seems). I'm sure UA will be around. If you have a lot of domestic flying too, NW may be better as UA doesn't auto-upgrade elites domestically. But if you're going primarily international, go with UA as I think you'll have more upgrade opportunity.