After trying out YoBit,I'd want to start marginal trading.The usual bitcoins to alt and vica versa.The site must be very supportive,less document requirements and supports most of the altcoins.
Also I will be day trading at times or invest in a stable alt for a week/month.My main concern is support,they trading website must support most of the alts and definitely bots.
Poloniex does have a margin trading. Tho I have really no experience on that I think poloniex is the best trading site for it.
https://m.poloniex.com/support/aboutMarginTrading/Also they have an optio where you can lend from other peoples money to tart margin trading so it's a good opportunity for starters. Poloniex also have a high volume for alt trading compared to other exchangers.