I actually use both Zipcar and car2go, and find they complement each other pretty well. Car2go is good for things where you don’t need to transport anything but yourself (and possibly one other person) and expect to spend most of the time at your destination rather than traveling, and enables spontaneous decisions; Zipcar is good for transporting large things, making substantial grocery runs (i.e. a monthly trip to Costco for purchasing in bulk rather than weekly things like fresh fruit/vegetables), or whenever you expect to spend most of your trip traveling, or when you need to make reservations well in advance.
I actually use both Zipcar and car2go, and find they complement each other pretty well. Car2go is good for things where you don’t need to transport anything but yourself (and possibly one other person) and expect to spend most of the time at your destination rather than traveling, and enables spontaneous decisions; Zipcar is good for transporting large things, making substantial grocery runs (i.e. a monthly trip to Costco for purchasing in bulk rather than weekly things like fresh fruit/vegetables), or whenever you expect to spend most of your trip traveling, or when you need to make reservations well in advance.