I would say there is none.

In fact few years ago I was on AA (2 of 3 flights oped by BA) mileage ticket from Tampa to Warsaw.

Took AA in first to Toronto, then BA in first to London, which was delayed in Toronto because of some mechanical problem so I missed my connection to another BA flight from London to Warsaw. Since that was the last BA flight for the day to Warsaw, the nice BA rep walked me over to LOT CS desk (even though they are not a part of OneWorld) and got me on the next flight out in business class. LOT rep helped me through security fast lane and then a golf cart escort as the flight was about to leave.

My entire trip was delayed by about 2 hours but it didn't matter that it was a mileage ticket.

Linate is very close to Milan (a relatively short taxi ride). It's a small airport, and I prefer it over Malpensa. And the advantage of taking a BA flight is that you will arrive in and then depart from Terminal 5 as long as all the flights are really BA flights, so there are fewer places to get stuck, assuming your flights are on time. And I think it's a huge advantage to be able to leave from DC rather than have to connect in NYC.

In my experience if using miles AA won't book you on a codeshare; however they'll happily book you on the same flights using the native BA codes if space is available. Codeshares are a selling device, not a redemption one.

Regardless, using BA metal will expose you to BA fuel surcharges, which will add significantly to the dollar cost as well as the mileage spend.

As for Milan, BA has more flights to Linate than Malpensa, so if you should be late there are more backups. On the other hand, LIN is on the opposite side of the city from Lake Maggiore, so if you're driving it will be a longer schlep. If you use miles, you wouldn't be bound by AA flight numbers that would push you to Linate; you could just opt for a MXP flight from Heathrow instead, even if it means waiting an extra hour or so at LHR, time you'd save at the other end.

Check with AAFrequentFlyer's response.

AA won't / can't book you on BA from the States using FF awards. However, they can from Canada.

A few years ago I was trying to book a flight to Europe and the Platinum Travel Desk told me about this BA requirement and routed me through Toronto.

AA won't / can't book you on BA from the States using FF awards. However, they can from Canada.

Since the US government granted Anti-Trust Indemnification (ATI) to AA and BA two years ago, that former prohibition is gone. One can now book BA with AA miles on any flight they operate, and BA FFers likewise on AA flights. In fact, AA, BA, and Iberia (IB) all codeshare on just about every transatlantic flight any of them operate.

Meant to add, I got off the phone with AA ten minutes ago having just booked a Helsinki-Oslo-London-Seattle itinerary for ten days from now. Finnair to Oslo, BA the rest of the way. $400 for fuel surcharges, by the way.

I had a nice (really) conversation with an AA advantage counselor today. She said the BA, AA and Iberia options were all open. However there is a great cost differential. Taxes, surcharges etc on BA would be about $500 more R/T than AA. Iberia would mean two stops rather than one. I'll take a look to see if there is any advantage flying out of Philly. BA does have easily the best schedule though.