I can find many arguments that will proove that, the first one is that the Poloniex plays the biggest part in the BTC market, and also very big part in the markets of some specific altcoins.
Also, we have the highest volume all over the exchanges at Poloniex, what makes their rates for buying/selling really attractive for the customers.
Another advantage of Poloniex is letting users to do margin trading ( even without ID authentication ), or also you can use your money for lending purposes, the rates are really good.
In summary, Poloniex just seems as a better choice than Bittrex, you have more possibilities with that exchange.
What poloniex offers make them top tier, their site performance is really outstanding that makes them also unique. Overall this is just a comparison and there is no winner here, it is just based on the people's votes and choices to which they prefer. For me both are good and unique but most of all they are trusted and reliable.