If you want some advice in this group of community,all you can hear have a different opinion in each of every member of this group,but in my own if i choosen this two, i rather to open a wallet of each coin.because if you put it in your MEW wallet you have your own authority to see or view your balances  in your own private key with your own risk,unlike if you put it in exchanges trading theres a big chances that it will loose,if those exchange trading will be hacked or closing there operation,just like happen one trading site in japan,while talking about wallet there is a most secure wallet they provide the metamask,or the ledger Nano S or this trezor,but this wallet you can buy with high prices at least all your coins is safe and secured.