It looks like you already made your choice, but just in case...

I would highly recommend you choose AA (OneWorld) or United (* Alliance). DL is IMO one of the worst program, and is part of the only alliance without a hub in ORD. Unless you are purposely trying to rack up segments (which it sounds like you aren't with 2 trips/week) or are going to ATL/SLC 50% of the time, stick with an ORD based carrier.

Now between AA and UA, AA will likely have a huge advantage if your company requires you to fly cheapest coach internationally. This is because as an EXP you will get SWUs which will allow you to upgrade virtually any purchased fare to the next class of service. United's SWUs can only be applied to certain (more expensive) fares.

If that is not a major issue for you, I would look into the routing options. If you plan to be visiting Germany and Dubai more, UA has the advantage. You can fly to Germany non-stop on several UA or LH flights. Heading to Dubai, UA flies on their own metal via IAD, or you can take one of several *A partners (such as Turkish Air).

Take a look at the route maps for both airlines and think about which airline will serve you best for your current and future travels. Once you choose an airline/alliance, stick with it unless the price or routing is just atrocious.