Well, I suppose the final decision must be based on how easy it is to get an award ticket with both programs, and which airline you will fly the most, as some benefits (like upgrades, one-ways, probably award seats availability) are better if you fly the airline whose program you are member of. I do not know about OnePass, availability in FB for my trips is ok, if I am a bit flexible (I do not fly North America - Europe though). NW has a bit closer relationships with KLM (and FB) than with the rest of the Skyteam, so if you fly mostly NW, FB is a plus. FB also has these WebAwards, where on selected destinations you can fly just with 50% of the miles required.

The taxes on FB will be something like Lufthansa's, yes. For short trips comes about 50% of the price or the paid ticket. Flying from North America to Europe, with luck and careful planning you can get quite cheap tickets, so the taxes can seem high too. The problem is that even if you fly CO metal and pay with FB miles, you will pay ticket surcharges to FB just as if you flew AF. There is also the problem that an FB award ticket in Business is 90000 miles, in First 100000, a very small difference, but availability in First is low.
For me personally, if I were based in North America, I would choose a north american airline.