Quote from FastandFurious:
so use ECNs?
Not exactly. You're at a chop shop, so they're probably charging you .0075-.008 all in, so it doesn't matter. But if you get out of there someday you will see the pricing difference between ECNs and DOT varies for things like adding and taking liquidity. It's still more efficient to send your orders to DOT when taking liquidity, but it all really depends on the situation you're in at the moment. If you're going to get spread up/down 3 cents on a market order with the specialist, it doesn't matter if quickly hitting arca to cover costs more per transaction.