If you want to fly AF F you must have your points in FlyingBlue, so that is a consideration. On the down side, booking with FB will require fuel surcharges to be paid. Transferring the points to a US-based carrier (CO/DL) will require you to pay the excise tax (up to $50 on a transfer) while transferring to the foreign carrier will not have that tax.

And the 1800 miles you have now are not worth worrying about orphaning.

I really wouldn't move them until you are ready to actually redeem. They are way more fungible in the MR program than they are after they've been moved to an airline.