Open.BTC is a token that runs on BitShares platform that claims to be redeemable for bitcoin.
BTC-U is a token that runs on the BitShares platform that claims to BE bitcoin - moved to a new platform.

BTC-U is one of three implementations of the bitcoin ledger at a point in time that evolve independently from that common branch point. Since it is the same owners of the same tokens in all three cases, all are equally legitimate. The only question is which accounting system people prefer to use for those tokens from that point on.