In terms of interface for user and all, Bittrex is better to poloniex. It is easier for you to navigate and the bittrex design is easier on the eye. Apart from this the response time and speed of trade execution are not different, may be poloniex is even better in this regard!
but poloniex is more expensive in regards to transaction fees,this is most problems of the poloniex users,but the service is more engaged to the both parties,but for me these two are good in their own style of service,
For me bittrex is somehow good in earlier days when verification is not yet required but when they impose the verification its pretty hard for me to get verified although i just pass them my requirements.Since i cannot trade on bittrex i just look for another exchange for my trading activity.By the way bittrex has a lot of volume and many user doing trading there.
You got a point there mate,in early years bittrex is so much better and has been supported by many,but these days ive been hearing lots of problem about this and the response team is not enough