Start with four characteristics
When comparing cars, avoid judging them only by how fast they feel. A useful practice car should give you feedback that is easy to recognize and repeat.
1. Steering response
Notice how much the car changes direction after a small steering input. If the response is extremely sharp, fine corrections may require more precision than you currently have.
2. Power delivery
More power can create larger mistakes as well as faster acceleration. If the rear constantly breaks traction when you apply throttle, it can become difficult to tell whether the problem is your input or the setup.
3. Weight and rotation
Pay attention to what happens when you brake, lift or change direction. The important part is not eliminating movement; it is learning to predict the movement.
4. Repeatability
Drive the same section several times. If the car behaves differently because your inputs are inconsistent, practice first. If the car itself produces an unusually difficult response, that is useful information when deciding whether to change the build.
When should you switch?
Switch when you have a specific reason. You may want a different power level, a different balance or a car that teaches a particular driving characteristic. Switching because of one poor run usually gives you less information than repeating the run.