Much better to have multiple trading site to have an option for withdrawing fund and to lessen the risk of my investments.
I also use poloniex for my trades, but be aware, you need to learn that you can't fully trust any exchange site, no matter how big they are, no matter what they promise. When something bad happen you will not have time to react and there are always chances for that. There are so many things that can go wrong with one exchange site, history should be a big teacher read more about what happened with some exchangers in the past and learn from that. No matter which exchange site you use don't store there all your funds.