Each type has certain advantages and disadvantages. Any business has a minimum amount of overhead that must be paid for out of sales. The goal is to grow as fast as possible to spread these costs out. If you have picked a good area, it may take 4-6 months for a vehicle to attract enough members to break even. Private businesses seek investors and pay dividends.
Some states allow coops to pay non-member investors dividends as well.
If you're leasing cars you'll probably have to carry comprehensive and collision as well. The insurance company will probably define the minimum driving record and want to screen your members. A mix of transit and bike usage for commute trips and pedestrian oriented services for shopping, entertainment etc. is probably more important than density alone. Maintenance, towing service and vehicle cleaning must also be planned. Most groups put a lock box on the wall nearby where the car is parked.