I have only been using Poloniex and Bittrex for about one month now. It is my experience that Poloniex has more issues to resolve and is also slow to respond to these issues. Bittrex has been problem free so far and the speed of exchanging funds has been excellent. Both sites are a bit confusing on the use of the exchange so you will need to watch several videos on how to use them. Some cryptos are only available on one or the other so you may have to use them both if you want the most exposure to all the different cryptocurrencies.