Delta coupons look cheaper than American's, at 10K miles for 3,200 miles of upgrades vs. 30K for 4,000 on AA, but you can't use them with most discount fares. They're effectively upgrades from full coach, or within shouting distance of it. American's upgrade from any fare. In principle coupons you can use with any fare are worth more, though if one is already paying enough the Delta upgrades are a better deal on that specific flight.