DL would be a better choice than CO if you're interested in system wide upgrades. DL awards 6 such certificates per year (though you have to buy the more expensive coach fares if you want to use them.) I have also found it much easier to get upgrades on DL domestic flights than on those of Continental. In fact, as a DL Gold, I had a better upgrade percentage than a CO Platinum. If upgrades and system wide upgrades are important, I'd avoid CO and favor American and/or Delta.