Two side wheels, one front (or rear) caster

This is the most common powertrain arrangement because of its simplicity. Only two drive motors, and no steering motors are needed. Wheels can be mounted directly to the motor axle (in most cases) to avoid any complex gearing or belt-drive assemblies. However, when accelerating, the weight naturally shifts to the rear of the mouse, and when decelerating the weight shifts to the front. This means that when braking the traction of the tires is reduced, reducing the overall braking performance. Of course, with encoders it's concievable to implement some simple anti-lock brake system.

Two side wheels, double (front and rear) casters

This design isn't good because it's easy to accidentally lose traction with even the slightest variance in ground height. Ideally there should be no practical conditions where neither tire is in contact with the ground, but this design cannot guarantee that. Mars I is a clean example of this approach. It's possible to put either the drive wheels or the casters on some sort of simple suspension, so that if the entire weight of the robot rests on just two of the four tires, the suspension will lower enough so that a third or fourth contact point is in place. Fine tuning springs and levers at this scale is difficult, however, due to the weight of the mouse.

Two side wheels, balancing (segway-style!)

High on the wow-factor but low everywhere else. However, there are some clear advantages - without a need for casters, you are guaranteed that both tires will always have full traction. Control loop frequencies must be high to stabalize the chassis, and some gravitational feedback (such as a gyro or accelerometer) is required.

Tank-drive style

Turning is less deterministic because the wheels necessarily skid when they are not all driving with the same velocity. However, in-place turns and sharp corners are possible. The powertrain design is also the simplest of all the four-wheel designs.

Inch-worm style

Turning is accomplished by 'bending' the body of the mouse and having independent speed control on all four motors. This steering method, combined with independent speed control, avoids skid-steering but it is unclear if steering is repeatable under all conditions.

Train-car style

This design is similar to a conventional vehicle in that the front and rear wheels are bound by the front and rear axles, respectively. Either separate wheel motors or a front and rear differential are required to avoid skid-steering. This is the most mechanically complex of the designs but affords great speed and maneuverability.

Mars Rover style

This design is the ultimate in flexibility. It uses four independent wheels with steering motors for each wheel. Thus the robot can follow any path without skidding, and is able to adpat to curvature changes deterministically. Mitee VII has demonstrated this design elegantly.

Three-wheeled triangular wheelbase with roller-style tires.