I prefered poloniex, cause the interface looks much better.
But few months ago problems started at poloniex with incredible delays and timeout errors when trying to place orders.
Also support responses took very long.

When poloniex was not usable at all anymore, because of the delays, I switched over to bittrex.
Everything works fine and the support was very fast, meanwhile it is also slow (because of all new customers), but it is still fast enough (few days).

I can not explain it, but I have the feeling that poloniex looks much more professional, while bittrex is more amateur. So I would trust poloniex with bigger sums, than I do with bittrex.
Poloniex is useable again, but sometimes there is still small delay, therefore I stick more to bittrex for now.