I've actually decided to do much the same thing with PhillyCarShare, and sell my car.

Additionally...does zipcar provide insurance in the cost of rental? That was the final tipping point for me, realizing that I'd be shelling out circa $300/mo for insurance for my car here and couldn't reasonably run up that high a bill on the CarShare.

Anyhow, dunno if that factored into your figures.

~Blaze