After driving both cars the B4 is the simplest car to adjust on, and just about everyone has one so parts can be easy to find at the track, and AE parts can be found at just about every hobby shop if not all of them.
With the RB5 you will have to order most of your part on-line (a-main, and speed tech are great places to go to for that), and being that there is a little more things that you can change it can make the car a little harder to adjust, but I do like that there is 2 differnt rear ends for the car to fit your driving style.
On track performacne the B4 has a lot of steering, and the rear end is on the loose side, and that makes it a little harder to drive when the traction is low. Also the B4 isn't the best jumper out of the 2, although throttle controll in the air will fix a slight nose up or down. The B4 is a great car, but can be hard to drive at times (and with my driving style I drive very agressive (and at times I will crash. lol
As for the RB5, it has a good amount of steering, and the car rotates really good, the car is smooth on the track, and it has a little of the rear end grip of a Losi (it is the best of the AE, and Losi in a way). The RB5 is easy to drive fast, even when you are just putting around the track it is a fast car.
So for me I really love the way the RB5 drives, but at the same time I like the B4 when on a high grip surface as it has a ton of steering. The bigest thing for me is that I can push the RB5 a little more then what I can with a B4, although both cars are really fast on the track.
Is the B4 or RB5 the best car for you, no one knows but you. But I can tell you that the RB5 will be a little easier to drive then the B4, but at the same time parts are harder to find for the RB5. If you can drive both cars, give them a go to see what feels right for you.