I think it is better to store your coins in a wallet. Once a trading platform is hacked say goodbye to your  coins. Though having your coins in an exchange is quite advantageous too, let's say the price of the coin suddenly went up, you can easily trade it. Having it in your wallet, it will need quite some time to be confirmed and during this time price might have already dropped.
If you're in with holding the coin for a long time, store it in a wallet.