I've used Bittrex for the 3 months i've been trading and just recently got into Poloniex. I never had a problem on Bittrex but on Polo i've had my account locked out for no reason and its been too laggy to use on laptop. IMO both have their place. Poloniex lets you margin trade on the big market cap coins like Ethereum and Ripple. Whenever a new partnership is announced placing a long on one of these is a missed oppurtunity if you don't take it. I also feel Poloniex has a better layout and less distraction from low market cap coins. If I want to invest in a low market cap coin I use my balance on Bittrex but sometimes you get burned trying to chase all pump and dumps going on there and you need a site free from the madness- Polo provides that.