Welcome!

One word of advice - you state in your first post that it "It seems like you can basically fly on any airline and transfer credit to each other"

I just wanted to clarify that you dont necessarily transfer your mileage back and forth, you pick a program before departure and credit your mileage there. For instance, United is a partner with US Airways. If you fly on US Airways, you can choose, prior to departure to deposit the mileage earned into United because they are partners. You cannot, however, fly US Airways, deposit the mileage on US Airways and then at a later date transfer your miles to United when you want to redeem an award. Transfers between airlines can sometimes be done, but not without great devaluation. I apologize if you already knew this - I may have misinterpreted your first post.

I agree with the advice earlier, it looks like UA or CO would be the choice for you. After late October they will both be in Star Alliance. In that case, pick one of them as your main FFP, and anytime you fly UA or CO (or any other Star carrier) credit the mileage to that account.