if you see yourself flying CO a bit, then BD makes no sense IMO. CO will earn 100% on BD as well, but CO has better Elite bonus and you will get your domestic upgrades. With BD you could use the UG coupons and get redemptions in C for the Y price, if you see yourself flying in Europe a bit.
I would decide based on the projected flying pattern. If you will have a lot of intra-Europe flights (e.g. on LH), then CO might be great; if you have a lot of HUAUP flights, then BD is better. Why not maintain both? BD is easy to get and CO Plat isn't too bad either.