Well a big question is, how do you accumulate miles as it is? Just by flying? Or do you use affiliated credit cards or some other method? And is status a factor? If you're traveling a lot on AA/US, one would assume you have some sort of elite status that needs to be maintained every year - IF the benefits of elite status - upgrades, baggage allowances, lounge access... matter to you.

If you DO have to maintain AA status by flying BIS miles, then the question becomes HOW you'd "diversify."

Where's your home airport? One benefit of AS, of course, is their range of earn/burn partners - from AA and several Oneworld members to DL and several Skyteam ones, but also Emirates et al. But if you don't actually fly on AS metal, then elite status, lounge access, etc. - is a moot point.

If I were in your shoes, and - really - not knowing your flying patterns or other details - I'd probably get a Starwood credit card and go with SPG points. You'll get a nice signup bonus, and can accrue points in the typical way - HOWEVER - you have up to 30-something airline FFPs into which you can transfer points at 1:1 (and sometimes better) for redemptions, including several Oneworld, Skyteam, and Star Alliance members as well as several independents like Virgin Atlantic, Alaska, Emirates etc. A big one is Singapore Krisflyer, hard tickets to get from other partners' mileage plans.

That would allow you to have your cake and eat it, in a sense. Accumulate enough points to where you have some award clout, then transfer the SPG points to an airline FFP, and off you go. Note the transfers aren't instantaneous, so SOME degree of premeditation is needed, but the flexibility is certainly there. Good for hotel stays, too, by the way.

Hope this helps.