Comment by matwood

AUM, managing high wealth clients, running their own funds with expense ratios (also some of the lowest in the industry), uninvested cash, etc... Retail trading is commoditized now.

Anyone who really cares about spreads will be using limit orders. Otherwise you're talking about pennies on highly liquid shares.

The fact that we're even discussing the possible spread differences between market makers shows just how commoditized retail trading has become.