Poloniex and bitfinex have a larger trading volume than bittrex but both of them are risky to use right now.
I am using bittrex for both long and short term trading, trading volume they have is quite enough for me. Actually how much you are willing to day trade?
Small amount. Prob 250-500. What are the risk with both of them?
A lot of users have reported problems withdrawing alts from Poloniex with slow response times by their support.
Bitfinex got debanked earlier this year so the status of their solvency is unclear.