What is the difference between storing my USD/ETH/BTC on gdax and coinbase? I can transfer between the two, and my bank/card information on gdax comes from coinbase, but what's the difference?

In addition, I know there is a markup on Coinbase that can be avoided by purchasing on gdax instead. If i were to make the switch to Gemini, would I experience these same markups and if so how do I avoid them?