Continuous Track, as by their proper name were first seen commonly on tanks in WWI and so forth. Continuous Track is favored for it's large surface area and large amount of surface area and traction ruling in favor of large robots.

(working on it)

Wheel's are the staple of the Robotics community. Requiring only the most basic parts, wheels have been around for thousands of years. Wheel's have many advantages:

• Minimum amount of parts
• Large Maneuverability
  
• Simplicity
• Ease of rapid construction
• Easy to spot problems
• Good amount of traction
• Good for beginners
• Dependable
  

But then there are some disadvantages:

• Load of robot on axle's
• Load of robot falls onto the motors
• Prone to tip over depending on body, shape and structure
• To increase traction requires replacing the tire altogether

The wheel's simplicity mean that there are few parts that you must manage, and that causes for the wheel to be very dependable. You can easily throw some wheels onto any robot and make it work great, which also means for a simpler method of turning and spotting problems. Wheel's also have a respectable amount of traction when it comes to their size. There are downsides too. Poorly designed robots will have most if not all of the weight of the robot falling onto the wheel's and motor axles, wearing out the motors much faster (aka my line following robot spaghetti). Also robots can be more prone to tip over due to the low surface area, unless properly designed. Another problem is that to increase traction usually means to replace the entire wheel.

Based on the positive and negative points, I give wheels an overall     7
Some places where you can purchase some very good wheel/Gearbox combinations and their pictures and that I also own and use.

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(Taken directly off of www.pololu.com)

The Tamiya 70097 twin-motor gearbox is a small (3-inch long) plastic gearbox. It contains two small DC motors that drive separate 3mm hexagonal output shafts. There are two ways to put the kit together: with a high-speed 58:1 gear ratio or with a slower 203:1 gear ratio. Either way, the motors provide plenty of power to drive any small robot. For a gearbox with a similar form factor and more gear ratio options, see the Tamiya #70168 double gearbox.

The output shafts are 3 mm hexagonal axles that are 10 cm (about four inches) from tip to tip. The axles work with any of the Tamiya wheels we carry, giving you many options for your robot speed. The two low-voltage motors run on 3-6 volts and draw up to a few amps, making them perfect candidates for the Pololu low-voltage dual serial motor controller. The Pololu Orangutan LV-168 robot controller, which incorporates these low-voltage motor drivers, would be a great control center for a robot using these motors. Motor overheating can be caused by excessive stalling, even at very low voltages. We recommend that you use stall-detection sensors, or just watch your robot, to make sure that it doesn’t stall for more than a few seconds at a time.

The twin-motor gearbox is very similar to the Tamiya 70168 double gearbox, as shown in the picture to the right. The gear ratio options of the two products complement each other well, but the mounting holes and overall dimensions vary slightly. The double gearbox is shorter and wider than the twin-motor gearbox, and the gears are a bit smaller and wider. This should make the double gearbox a bit more robust, though we have not heard any reports of the twin-motor gearbox gears breaking.

For motor specs, see the Mabuchi motor FA-130 (#18100) data sheet (58k pdf). To compare all Tamiya gear box kits, see the Tamiya Gearbox Gear Ratio Comparison.

Tamiya Wheels

We have four different types of wheels from Tamiya’s educational construction series, each of which can be connected directly to the 3 mm hexagonal axles found in many of the Tamiya gearboxes. The sports tires and the narrow tires are the only wheels compatible with the 4 mm round shafts of the Tamiya “high efficiency” gearbox series. All of the wheels have plastic hubs and high-traction rubber tires; they differ mainly in size and springiness.

Tamiya 70111 Sports Tire Set (2 tires) The Tamiya 70111 sports tire set contains two attractive racing-style wheels, each 56 mm in diameter and 25 mm wide. These wheels are compatible with all Tamiya gearbox kits.

Tamiya 70145 Narrow Tire Set (2 tires) Use the pair of tires in the Tamiya 70145 narrow tire set for small and medium-sized robots that must make precise movements. With a 58 mm diameter and solid, rounded, 16 mm treads, these wheels will let your robot turn with little friction, even on rough surfaces like carpet. These wheels are compatible with all Tamiya gearbox kits.

Tamiya 70096 Off-Road Tires (2 tires) The two extremely springy tires in the Tamiya 70096 off-road tires set are about 50 mm in diameter and 30 mm wide. Use these if you want your robot to have a soft, air-cushioned ride across rough terrain. (Compatible with the Tamiya gearbox numbers 70093, 70097, 70103, and 70110, 70167, and 70168).

Tamiya 70101 Truck Tire Set (4 tires) The Tamiya 70101 truck tire set contains four 36 mm-diameter wheels with hard, rubber, 16 mm-wide treads. Our small round robot chassis is designed to use a pair of these wheels, which are great for making small robots! (Compatible with the Tamiya gearbox numbers 70093, 70097, 70103, and 70110, 70167, and 70168).

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Positives:

• Cheap good gearbox
• Hex axle is strong and can be adapted with other wheels
• Re-configurable
• Wheels are of high quality and are very nice
• Real rubber tires!
  

Negatives

• Gearbox is meticulous with many small parts
• Suprisingly hard to assemble
• Very loud
• Gears skip and create a very annoying screech
• Motor's can stall with 1.2A of current and smell when in use!
• Hex axle is hard to adapt to other things
• Hex axle can strip tire hex sockets
  

Overall, I would rate the Gearbox a 6, and the wheels a 9.

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(originally from www.solarbotics.com)

Need a gear motor and wheel? Get this convenient package, and save! Complete with a GM2/3/8/9/17 gear motor, wheel with molded-on tire, and mounting screw.(You get to select what color you want and what gear motor you want)

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Positives:

• Gearbox(es) are very high quality with internal clutch and are very quiet.
• Motors draw 100mA at most and stall with 500-700mA at most.
• Gearboxes are small and take up little space
• Gearboxes have double flat output shaft's with screw terminals making them easily adaptable
• Wheels are large and strong, and can grip very well.
• Wheels fit perfectly on output shaft

Negatives:

• Gearboxes can be slightly difficult to mount depending on the choice made.
• Wheel's use High friction rubber bands as the tire.
• Sometimes the motor terminals can become loose, but are easily replaceable making this more neutral.

Overall I would rate the Gearboxes a 9 and the Wheels a 8.

If you want optimum performance, I would select the GM_ Motors from Solarbotics.