<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Panosh42:
I am a leisure traveler. I hit mostly the resort towns, but want to go to Mexico City too. I am very price sensitive, I usually use Priceline or SkyAuction. I will typically go to Mexico/Caribbean once a year. I have a friend who works for Midwest so I fly domestically for cheap with them. I would gather miles only for international flights, maybe Hawaii someday.</font>
Well, if you use Priceline to bid on airline tickets, you won't be able to choose your carrier and you won't earn miles, so it's a non-issue. If, on the other hand you use a conventional booking engine (e.g. Travelocity or the airline itself), you have mileage accrual potential. But if your sole source of frequent flier miles is a once a year trip to Mexico, then you have a long, long time before you will even be in the neighborhood of the mileage required to visit Hawaii or go overseas.
Since you are unlikely to qualify for elite status based on your infrequent flying, forget about the perks of elite status (E+ seating on UA; free space-available upgrades within the lower 48 states and Mexico on CO). Without status, there's not much to differentiate between UA and CO in terms of their coach product. [I guess it's worth pointing out that you can earn CO miles on NW flights (and vice versa), giving you more options.] So just choose whatever carrier gives you the most convenient connections and, above all, price.
Meanwhile, if you want to pick up miles from non-flight activity, they are readily available. For example, by getting a FirstUSA Mileage Plus Visa card, you can earn 20,000+ UA miles without even taking a flight.