Both are very good exchanges, are popular and used by many. The volume of transactions as well as the user density of both are very high, both of which also support a wide variety of coin to attract many users. I think the choice depends on each person. For me, I choose bittrex, I feel trade with bittrex is quite friendly and easy to use. The security level of bittrex is good, it's quite safe. The support team is great, when there are problems we will be resolved in the best way and I like this. This is just my feeling