I'm always suspicious now when a broker sells order flow. I had the opportunity to compare some executions between IB and another well known retail broker. When I placed a limit order with the retail broker to buy or sell between the posted bid and ask prices, my order was never displayed to the market. That is, the bid and ask prices remained unchanged. When next there was an execution above my sell price (or below in the case of a buy), my order went through at my posted price. I almost never received price improvement. On one 500 share purchase of an NYSE listed stock, the difference between the 2 brokers was $0.05/share.
My point is - once you get down to 1 cent per share or less, execution is everything. I no longer trade anything significant with a broker that doesn't allow me to select the exchange where the trade will be executed.