FlyingBlue has switched to revenue based system, so it's likely you won't get as good a return on that ticket on FB. Redemption is undergoing a change right now, it's hard to say how it will stabilize. Apparently there will always be availability going forward, but pricing will be dynamic so it might be a losing proposition for you compared to Skymiles even disregarding earning differences.

FB 2 years expiration gets reset with activity, besides, it looks like your activity will keep you at Silver at least, with no expiry.