For the highly-liquid-on-the-ECNs instruments like QQQ/SPY/DIA, you'd do best to ignore NYSE and AMEX completely. The quotes are hopelessly out of step with the real market and you can't get a fill against them anyway. If you send them an order, and the market turns and goes against you, they'll ignore your cancel and fill you. That's why there's no arb opportunity. The specs can see the ECNs too (or have the equivalent knowledge of where the real market is), and aren't going to give you anything for free.