With Uber and Lyft on the scene I seriously question the sustainability of these services. I've personally replaced some trips I would have previously made in a Zipcar with Uber or Lyft. For trips longer than a few hours I just use a regular rental car. Because you have to pay for the time you reserved you wind up overpaying most of the time with car sharing and the rates are not especially cheap.
The problem with these companies fundamentally is that their customers don't respect each other. I've cleaned some previous renter's disgusting fast food garbage out of a car. I've gotten in a car that reeked of cigarette. I've driven a car covered in dog fur. These things are all explicitly banned but people don't care and the car sharing companies are probably terrified of going after people and alienating even more customers.
To answer your question, I don't think either is inherently better than the other, but in some places Zipcar has much better coverage than Enterprise so the convenience factor may win.