Poloniex is harder, Bittrex is easier. There are those who have moved from Poloniex to Bittrex recently because the terms of service update but I tend to think they are over-reacting as every exchange has the same policies. Some claim poloniex doesn't process their withdraws however to this day I have had no issues with getting a deposit or withdraw credited automatically in a timely manner.

I think that may have actually been a propaganda campaign to get new cryptocurrency users to go onto Bittrex which has a great number of outright scamcoins, it's easier to convince people to buy things on a whim when their funds are already in house. Since 2014 Bittrex has been listing and delistinig literally hundreds of coins which are no longer even listed on coinmarketcap. I think they are a legitimate exchange but that's just how it is.