Flying Blue is an unmitigated disaster. A quick peek at the FB forum will tell you all you need to know. No points for cheap fares, no upgrades from anything but full-fare economy (Y or B), and worst of all the incompetence of Flying Dutchman and the contempt of F+ seem to have met somewhere over Belgium in a kind of customer-averse perfect storm capable of driving away even the most adamant KLM supporters (like I was). Disclaimer: former F+ members don't seem to be notice much difference.

The one upside is that you'll get lounge access if you have a European address, and the newly-refurbished Crown Lounge in AMS looks to be quite nice. I'm also around 50/50 on long-haul op-ups flying on high economy fares, but this is due to careful planning.

On CO, you'll get domestic US upgrades. If you're doing much domestic flying (or connecting), go with CO for the upgrades, higher mileage earning, and better access to long-haul upgrades (providing they still do the hokey-pokey or something similar).