Shapeshift is better. Also may I ask which coins are you really exchanging or willing to exchange ? Because you can also look after the simple Swapping provided by the myethrwallet. I know it only gives you chance to exchange with ETH itself but if you are willing to convert thing for the final trade in the BTC and ETH then this is the best option for you. I am using the SWAP always and it does the job within matter of transaction confirmation. So it is always better to use it as its secure and they have proper support to if anytime your swap fails or have any problem with it.