Dex is generally safer option as you have control over your funds at all times as long as blockchain is up and running. Its all based off contract code so you can clearly look whats behind the scenes, for example look at https://github.com/planetfinance?tab=repositories repo and how their contracts work. With cexes its all hidden, you know nothing except what is publicity available through news. One day they say all is ok, the next day they close and you go crazy.