i love use poloniex and bittrex exchanges,its nice to start trade there,the feature just complete for me,huge amount of users,active market,and for poloniex,it have special feature,lending feature will love by everyone,for bittrex,i just not too happy with witdraw fee,its higher than poloniex 
Same here, ATM is the exchange that i used mostly is polo, bittrex is not bad at all but i prefer to use polo, also yes they added only few weeks ago the "lending" chooise, what i dont like from there is his chat system, sometimes is too laggy, like his charts scripts, but in general is the best, at least for me.
Another one thing that i would like to see from polo is to improve his API system, for php users especially you can feel like you miss some extra commands features.