Wow tough call - so many mediocre choices

I think the key questions for you are how you plan to accumulate miles and how you plan to spend them.

If you will accumulate by flying domestic economy class on UA then you might prefer UA as your status will get you E+ seating and some other benefits at the airport.

If you will accumulate by flying international (any class) you might be best off crediting to BMI as you will get some nice bonuses. However if you are flying UA and will hit 100k then you might appreciate the SWUs.

If you will be redeeming for domestic flights within USA then UA is the best choice as redemption is usally pretty easy and much better than US.

However if you will be redeeming for international business class travel then your best choice would be BMI, and to a lesser extend US. BMI (and US?) require fewer miles, but more importantly, neither appears to engage in the horrific *net blocking.

I see no reason to credit miles to UA anymore until UA decides to honor its obligations to provide saver international business class awards on both UA and *A partners.