I was considering poloniex myself, as well as cex and coinbase. Now I am not sure about coinbase, of course. They definitely have hard times. So there are two options left - poloniex and cex.
As far as I know poloniex is among the pioneers for offering margin trading and must have won the audiences trust. But what attracted me on cex.io is that they claim to use a risk-preventing system that protects your balance from becoming negative.
Both poloniex and cex have this same risk-preventing system (some method of Proof of Solvency) - in fact, I would assume all platforms that offer margin trading have it, since it is firstly to protect themselves and the lenders, in the case of Polo.
With crypto markets extremely volatile, I would highly recommend a practice account on a forex platform first, using the same leverage, before ever trying out crypto.