Well, firstly, counterparty risk isn't quite what you think it might be, as OTC FX options are collateralized. As to the cost of carry, it has to be the same, otherwise there's an arb. The payout of carry also can't be different for the same reason. Furthermore, in terms of capital usage, because exchange margins are a lot more punitive than OTC, your return on capital is going to be worse for exch-traded stuff. That's the price to pay for the "safety" aspect the exchanges offer.