What makes polo better than bittrex in my opinion has a whole lot to do with responsiveness and reliability.
I've had tickets on polo that were answered in less than 10 minutes. I don't recall bittrex answering my support ticket in less than 3 days. These aren't particularly complex tickets either. More like "Hey I made a deposit, it's been hours, where's my money?" that are basically the norm for every exchange.

Polo also has better volume and orderbook depth than bittrex usually.