Honestly it is not about one is better than the other, they both have their own good parts and bad parts. A decentralized exchange like shapeshift will allow you to keep your coins on your own wallet and this is a lot better for security reasons but you will have to have your own wallet on your pc or somewhere which creates added work flow and you can't use bots or whatever on stuff like that.

However, on centralized places you are basically trusting the exchanges with your coins and they keep it for you but you do not own the seed of your wallet which means you are basically not the owner of your coins, you are just at the hands of that exchange however it is easy to just simply buy and sell directly which makes it faster. So it depends, one is good on security, the other is good on fast and ease of use.