You don't need a car to get around Las Vegas, there's a good network of free shuttles and cabs are everywhere. But, a car gives you the ultimate freedom to come and go as you please. We recommend TravelNow because they offer the cheapest rental rates we have seen online, and you can compare rates by car-class and by company.
Taxis add up quickly.
A trip from the Strip to downtown and then returning will cost you more than a cheap rental car. A bus trip will cost more than a rental car, and will be less pleasant. There are shuttles, but they don't come and go according to your schedule. There is plenty of free parking everywhere, and Las Vegas is easy to navigate. Just stay off the Strip in the evenings and you'll avoid the worst of the traffic.