Concentrating on a single chassis is key if you really want to improve. You can learn alot about how the car reacts if you continue to run just the one car. I've only had three different cars I've really raced on road since I started. I started with a TC3 and it just didn't fit my driving style. Went to Losi and liked the car but about 8 months ago found out about the new car so I started looking for other options and got a chance to run for Shumacher and am very happy with the decision.
But I do think what ever car you run you should stick with it if you like it to start and just concentrate on that and continue to make it faster.