here : http://bitshares-x.info/  it is absolutely brilliant and it only came out like a few weeks ago.

Personally I would recommend bitfinex, and am really not familiar with okcoin. There are some people that think the amount of leverage at bitfinex (in total) is a sign of an overextended market that would be subject to a flash crash. People also speculate that lenders on bitfinex would be left holding the bag in this event.