I used to be based in Europe but now fly to Eastern Europe from LA. I do 4-5 trips/yr to Europe; only 1-2 in the US. So I use Frequence Plus as my primary FF program. Reasons?
1. SkyTeam Elite Plus status (lounge access) for only 40K miles.
2. Since LAX has lots of non-stops to Europe by the European carriers, I have the possibility of upgrading on Transatlantic and European flights. (OK, AF isn't great for this, but if I think Miles and More would be better than Mileage Plus if I was primarily flying Lufthansa.)
3. I think it's easier to book awards to Europe (for friends and family) with the European carriers. At least, I haven't had any trouble over the last few summers (booking only a month or two in advance); on the other hand, trying to book even nine or ten months out to Eastern Europe using my wife's Continental miles has proved fruitless.
So I think it all depends on your travel patterns.