I think AA's recent trend to open up more J/F international awards on their own metal makes me also agree that buying the tickets is a better option. I've found AA international premium hard products are pretty nice (except for old slanted J). If down the road you can use those miles for their 777-300 J product while paying cash now i think that's the best use.
I will say I just did ORD-LHR rt in BA J and was pretty happy with the seat, considering the bashing BA seems to get on these boards. If you keep the original itinerary I think the hard product will get you there decently rested.