So as I understand things as an AF/KL FB member:

-I will achieve elite status faster than CO OnePass
-I can use the priority security line on domestic flights as an elite.
-CO and DL flights will give me the full mile value (they will not be EQM though)
-I will not be able to use CO lounges as an FB gold (elite) when flying on a CO ticket (I don't like that)
-I will have access to NW lounges as an ST elite. And of course AF/KL lounges.
-I would not get upgrades on domestic CO flights.

The things that are most important to me in order are

1. Security! (Avoiding that line as much as possible) (As a former air line empoyee I know that that will be the hardest part of my transition back to a normal passenger.)
2. Upgrades
3. Lounge Access
4. Redeeming Miles

So it would seem that I should go with FB because security is my primary concern.