Ohh, trust me I don't want to start a debate over drive motors. You perfer the CIM, I like the drill, both have their plusses and minuses. But I would like to make a point on where you put your casters, if you look under the bot my team built last year, you will see some pretty meaty casters.
Here is a view under Sparky. While we have not always used casters (Sparky v3.0 was 4wd), we like them because the feel easier to drive with, if you have the proper casters. But still go with whatever your driver is comfortable with.
Ivey