Not a clue if they are still valid or not, but the Hertz coupon thread has some free membership application fee waived codes in it from when connect was starting.

I have heard good things about both, one friend prefers zip because they can use their iphone to get the car. I would base it on which is a better price and closer to where you need to go.

I also believe both connect and zip run discounts through their facebook and twitter feeds from time to time, so follow them.