Cheap fares on DL are non-upgradeable, even domestically. To UG on DL int'l, you have to be on a Y/B/M fare, which to europe will always be over $1,000USD.

Almost all fares on AA are upgradeable. Even most consolidator fares that aren't elligeable for mile accumulation are elligable for upgrades on AA. The only exception is certain web fares available at aa.com.