GDAX is a sleek professional trading platform created by Coinbase. The automatic account integration with Coinbase makes it easy to transfer funds and adjust profile settings. The platform is incredibly simple to use and combines rich trading features with an intuitive interface. I highly recommend GDAX for anyone already using Coinbase.
Kraken offers great volume, responsive customer support and many advanced trading features. My review left me with some technical concerns and it sounds like I'm not the only one. If you're just starting out with trading cryptocurrencies, maybe start with a more beginner-friendly platform.