For liquid stocks, does it really matter whether you end up routing the order to an ECN or an MM? I know that ECNs give rebates for adding liquidity, but the benefit of this usually only really applies to heavy volume traders or prop. traders. Also, according to what I've read people are using market orders to ensure that they capture a market movement anyway. Basic proposition is: In most cases, SMART routing through a decent broker is sufficient (best execution should be at the bid/ask). Question is: When is it NOT sufficient? Only when "scraping pennies", or is it generally insufficient? Why would one route to a specific ECN or MM when there is usually additional cost to do so?