Usually you want a good torque servo as your steering servo to move those big tires left and right, and you want a high speed servo for your throttle/brake. I have been running the Hitec 7955 for my steering and just a decent speed servo for my throttle/brake. I have used the Hitec 5955 and 7955 for just over two years with no problems. They are a little pricey, but I have had zero issues with them. In my opinion, spending a little more for servos in the beginning will save money in the long run. I have tried airtronics and cheaper JR servos before, but they ended up dying on me anyway. Another one to look at is the JR9100T, I have one in my X-ray and have had no issues with it either.
Mike