City CarShare and ZipCar have very different pricing models, and which one is best depends on how you intend to use it. I hesitate to give specific advice on that though because it has been a long time since I have researched them, and pricing structures have likely changed since then. With either one, for someone who doesn't drive much, the cost is going to be lower than owning a car, especially if you are paying for parking. Heck, it could be cheaper than just the parking alone.
The main down side to car share programs is that they are less convenient in some ways (though they are more convenient in others). Depending on where you live, one or the other may have more cars near you, and if so, definitely go with that one. I was fortunate enough to have a POD just a block away from home, closer than I was usually able to park my own car when I got one. So, depending on the particular circumstances, a shared car may be more convenient to park or less convenient. You definitely will always have a place to park it; there is no driving around looking for parking at home. Check for cars close to where you work, too. These days shared cars are all over the city, far, far more common than when I had a membership, so you may find both companies have many cars close to you, but it's definitely worth checking.
The greatest inconvenience is having to schedule your time with the car. If you want to use one to go shopping, you have to know how long you will need it. Courtesy (and rules enforced with fines) requires that you return the car on time, so you may need to reserve it for much longer than you need, and pay for that time whether you use it or not.
Often you can reserve a car at the last minute, but if you want a particular car or particular location, you'll need to plan in advance for that. I can't remember a time when I was not able to get a car when I needed one, though sometimes I had to pick one up from farther away. Certain times, like holidays, may create greater demand, so you need to book farther ahead to get the car you want when you want it.
One of the greatest things about CCS was that they had pickup trucks. (I'm sure ZipCar does too.) You never need to bug that friend with a truck to help you move something; you've got your "own" truck. Or Mini Cooper. Or Prius. The cars all tend to be fuel efficient, but other than that commonality there is a wide variety, and you can choose the right vehicle for the job.
Eventually I was kind of forced to buy a car, and although I would never, ever have done it otherwise, frankly I love owning one again. I picked a good car for the city (short) which makes a huge difference. I've had it for about 2.5 years now, and even though I am not commuting by car anymore, I have no intention of giving up the convenience of being able to drive whenever and wherever I want to, for as long as I want to, on as short of notice as I want to. At least, not any time soon.
But circumstances are different for different people. Since you are paying for parking I suspect you live close to downtown, or in another area where parking is highly impacted. That also likely means you live within walking distance of BART and/or CalTrain, or at least are a short Muni ride away. In those circumstances, I would absolutely sell the car and get a car sharing membership and establish a relationship with a rental car company, like a frequent renter card. (Rental cars are almost always the better option for trips out of the area, though both companies have options for "weekend away" type trips.)
Zipcar is simple, efficient and relatively affordable. I've actually found that Hertz is the cheapest for weekend trips.
If you don't drive that much, a Zip Car is a good idea if there is a close by and easily accessible
Zip Car Rental Lot. Check out http://www.zipcar.com - not only can you find your nearby Zip Car Lot...
in addition, under Rates & Plans - there are these helpful information:
1. Occasional Driving Plans
2. Extra Value Plans
3. Compare Plans
4. Savings versus Car Ownership
If you have a garage, it's easy to park your car even when not using, but if you don't have a garage,
it may be a deciding factor for a Zip Car.
Helen Yuen, Realtor Associate, Land & Property Investments SF (415) 469-0577
Please give me a call - Buy * Sell * Invest * Loan Info * Listings * Market Prices
The answer to your question comes down to personal choice. As an owner of a car in the city, you already know some of the drawbacks of having a car. Zip cars are convenient, cheap, and easy to deal with. But they can get pricey if you use them every day.