Two points:
1. AA's D3 passes for Europe travel are not that much less than a discount ticket that would have a confirmed seat.
2. AA D3 passes for International travel are not available during the Summer travel season. There is an embargo on D3 travel every year from about June 1st until September 1st (or maybe Labor Day).
I would urge you to look not at what any particular airline charges for non-rev space available (NRSA) travel on International flights in the summer vs. another airline's NRSA charges, but rather look at what any particular airline charges for NRSA vs. a confirmed seat discount coach ticket. Besides as with AA, I would be surprised if buddy passes are even available on United during Summer travel season.
And, with the capacity cuts that all airlines have made, assuming you manage to obtain a buddy pass on United, plan to spend a day or two in your departure hub (probably ORD) trying to get on a flight. My base manager and her husband went to Italy in late September last year on AA--not exactly what you would call high travel season. She had to ride the jumpseat all the way to Rome from ORD because there was only one seat available on the flight; so, they put her husband in it, and she rode the jumpseat.