If you travel on cheap fares with a mix of Trans Atlantic and European I find that:

- For Tran Atlantic United offers best mileage/upgrade propsect
- Within Europe there is often not much saivng for coach (expect using cheaper airlines such as Go). Here you may be better off using a European airline instead of using MP on Star. This is because accruing Star on United means using miles flown, which tend to be less in Europe.
- Use BA Amex card to pay for all fares (including United).

This works for me