UA is an obvious choice here because of VIE. With UA you have the option of the direct flight on OS or numerous one-stop options on UA/*A metal or all *A metal (FRA/MUC/LHR/CDG/ZRH/BRU/ARN/CPH/IST/WAW).

AA you are basically looking at LHR or MAD only for partners (maybe HEL or TXL for Finnair or airBerlin but inconclusive from a quick glance at google flights).

With both you'll need top tier status to upgrade with instruments and you can look on their websites for the cash + miles upgrade cost (can't recall it off the top of my head). With UA you have more options for potential upgrades (see list above where UA operates ORD - European City so you can upgrade on UA metal) whereas for AA you can only upgrade on the AA-operated ORD - LHR flights.