Will be traveling in Business Class either from PHL or IAD to FRA in early September. Want to take advantage of the current offers out there for a free Economy ticket with the purchase of a Business Class ticket. Not sure if I will book US or UAL for the promo since they both have direct flights. Can anyone recommend what they would do?

I know UA offers more destinations, which I can use later with the free world wide ticket but the problem is that I would have to open a UAL FF account since I currently put all my Star Alliance miles on my LH Senator card. (Promo requires miles to be credited to UA to get free ticket!) With US I am more limited as to destinations they fly directly. Promo states that US voucher can be used on UAL as well but I think this will be harder to do.

Recommendations Please!