oh, so you are ONLY flying to Europe. AF treats their Plats soso, there are no automatic upgrades, but on BOS-CDG you have a chance for an OPUP, since flights tend to be very full. You get lounge access with any ST Plat program, so you might want to focus on miles and bonus, which is better with NW. IMO AF will prefer own Elites when it comes to OPUPs, but there are reports that NW and others have been upgraded as well.
I personally find award availability decent on AF during normal season and actually quite good in Y during peak.