My preference is Bittrex. Polo has been very slow lately. I've also noticed that there is some lag when changing between pairs... the details of "My Open Orders" and "My Trades" don't update at the same time as the rest of the page. The chart might be showing one coin and the details at the bottom are for the previous coin you were looking at. This has almost led to me placing trades based on wrong pricing.
I find Bittrex slow but it may be just me. I never had that issue but glad you brought it up. I will start getting my coins into Bittrex and than to wallets