Neither are true. You definitely need speed. I had a slow servo Hitec 645MG. Almost impossible to drive my car at the track with this servo. The buggy was twice as fast as it could steer. The XP DS1015 broke on me. But I still stand behind them. Mine was a fluke. Torque 201 oz/in speed .108 sec to 60 degrees. This is a fast and powerful servo.

Now as far as wide chassis goes, the chassis should only have to do with the weight and design of the buggy. Not saying it doesnt have anything to do with handling at all, but the length of the suspension arms would play more of a role in the handling. Wide is good. Narrow and you're prone to flipping. I have never gone around the corner with mine and had it tumble from being too narrow. The RC8Be is a great handling buggy.