I would go with UA MP if I were you. Multiple flights and plenty of rerouting options. Since you'll be flying coach, you cannot beat having Economy Plus. You'd be surprised how much of a difference it makes! With being a UA MP member you'd have the option of flying on UA, LH (connect in FRA, MUC, or AMS), or BMI. That gives you three carriers that you can earn in MP. You also have AA and BA....only thing about one-world is that you don't earn AA when flying BA (if I remember correctly.) What would be your goal out of joining a frequent flier program? You might also look into the citi premierpass elite level and you would earn thankyou points (when charging the flight to your card) PLUS your frequent flier miles.
Halothane