Not long ago, Poloniex used to top coinmarketcap charts when it comes to altcoin trading but not anymore. Bittrex has taken its spot.
As of today, Bittrex has: $257,634,233 and Poloniex has: $114,189,318 (almost half of Bittrex) in 24 hour trading volume.
Bittrex is not only gaining volume on a daily basis, users are also shifting to it because they need a stable trading platform. In the past, Poloniex had been terrible, both in terms of platform stability and customer support response time. Normally, they would take anywhere from 5-7 days to reply.
Not to mention, there were instances where users reported missing coins from their wallet due to a bug in Poloniex's stop-loss feature. Poloniex has fixed that issue later, but apparently the damage has already been done.
No one would like to see their coins missing, right?
Based on experience; I wouldn't vouch for Bittrex in terms of customer support response time and or handling, but they're better than Poloniex for sure. That's probably the reason they've taken Poloniex's place.
Bittrex has approximately 200 different altcoins while Poloniex has a much less no# of altcoins one can trade (approximately 70 at the time of writing). Personally, I find Bittrex's interface better than Poloniex's.