First of all, poloniex is an extremely unsafe exchange to deal with at the moment because there are simply so many complaints about them having fake lag, delayed withdrawals and deposits(sometimes still unresolved till now), and unresponsive support. anything goes wrong with your account, you are essentially in an endless hole of contacting the support every single day with no response from them.

What you want to do seems to be day trading. Now, from my experience, coinbase is a reliable exchange. However, first, make sure that the source of your fund is legit. Otherwise, your account may become suspended. If you want to day trade, then you'd be hit by high fees charged by coinbase. They've got to make money, so they have pretty high spreads when compared to other exchanges and higher fees. I'd say, either try Kraken, or BTC-e if you want to trade short term. But don't hold too much in any exchange.