We rented a car and didn't use the busses at all, so I can't really compare. We did take the boat to DTD when we planned to drink, and that was great, so even if you rent a car, you might plan to use the busses then.
At any rate, we found glancing at the map they give you at check-in helped us find our way to the parks pretty easily. We really enjoyed having the car, and taking the trams, etc. always seemed faster than waiting for the busses. That may be particularly true for parks that have one big event after the park closes (like Fantasmic and Illuminations) since the entire park crowd tries to leave at the same time.
The main reason I rented the car is because I know my boyfriend hates crowds, and that loud people (especially kids) can be trying for him at the end of the day. After a day waiting in line at the parks, in line to eat, in line to even walk sometimes, not spending 30 minutes standing on a crowded bus was really helpful.
We got an economy car from Priceline for about $120 (after taxes and fees) for 10 days.