Poloniex is the best an exchanges in a few years ago. but at the moment many of trader is move to bittrex
I personally prefer to trade on bittrex right now.
Poloniex is older but I don't find it trustable. Bittrex seem to be more trustable to me. I ready many reporst about withdrawing problems happened in poloniex in the last years.
Hence, I think Bittrex is a better selection.