Best place to short bitcoins is bitfinex.com by far. The interest rate for margin when shorting bitcoins is so low that it is almost 0. bitfinex also offers wire deposits and cashouts. I can get you 5% lifetime rebate also via sig.
I've used the bitfinex loan system. Loaned people money for X days, got told this was the minimum term the loan would run for, then after X-lots days the loan just disappeared. When asked management they said they couldn't do anything about it. Kind of pissed me off to be honest. You can't have a minimum loan period which is barely ever honoured by the person borrowing the money.