I had to make the same decision for my trip from LAX to ATH this coming October.

Here is how I analyzed it:

BAEC miles to me are worth about two cents (U.S.) per mile. Therefore, the additional 50,000 miles for the two of us to fly F instead of CW (using a 2-for-1 voucher) "costs" $1,000. This translates into $250 per person per TATL flight to fly F instead of CW. The flight from LHR to ATH would be in CE either way so I ignored it in my analysis.

Personally, for me this is worth it but I don't think I would spend much more than that to fly F instead of CW. I have flown both CW and (old) F in the past two years. The main difference to me is that the sevice in F is more personable, the F cabin is quieter, and the food and especially the wine in F are far superior to CW. In 2009, they were serving very good and reasonably aged Bordeaux ($100 per bottle wine) in F. I don't know if this is still the case. For my 2010 flight in CW, the wine was unremarkable at best. I didn't find that much difference in the seating, and the IFE in CW is better than the IFE in (old) F.