I've got both UA and AA miles, and have checked availability with both. In my highly selective sample, looking solely for F awards, UA works best for Europe; AA works best for Latin America, and both are good in Asia. AA works well for Africa, with a combination of AA across the Atlantic, then BA onwards.

United has a lot of European partners. It is possible to string together a trip with 4 stops, as long as two are under 24 hours. LH F is excellent and Swiss is coming on board. If I use AA, I have to fly AA across the water, since BA can't be used for awards from the US. Then I have to use London as my hub, rather than being able to hop and skip across Europe using LH via FRA and MUC, SAS via CPH, OS via VIE, and more. It has been a great surprise to learn how many European cities have nonstop service using Star carriers.

Further, UA F is 100K for European redemption. It is 125K using AA miles. Making UA even more attractive this year is the benefit it has extended to top tier members, allowing changes with no fee until the day of travel.