there is no logical reason to limit yourself to one exchange or the other, unless you are some kind of highly specialized scalper, like you only scalp IBM all day long. And if you are doing that particular example, then you are competing directly with the floor traders who aren't paying any commissions and who can see order flow coming in. I think a lot of this listed vs. nasdaq bias is coming out of the firms. Some firms are setup to do one or the other, and they are biased in favor of the one they are setup for. Some prop firms are completely setup for adding liquidity to ECN's for example, obviously they will tell you that NYSE sucks. If a firm goes through all the trouble and expense to become self clearing on the NYSE, obviously they will want their traders to use that capability.