I prefer myself Bittrex over Poloniex. There is not any particular reason, it's just that the Poloniex UI hurt my eyes.It is confusing that you don't know where to put your eyes. Bittrex is also the first exchange I used and I am happy with it. I don't like to change specially when all is working good. So i stick with Bitrrex personnaly
Came here from a thread linked to from another, and found it funny that this was the first thing that struck me about Polo the first time too... the default UI hurts my old eyes. You know you can switch to Night Mode, though, right? (Top right corner)