The best option will depend on the type of terrain you expect to cover. You want the correct balance of several factors:
- ground pressure
Ground pressure from tracks is less than ground pressure than wheels, so they're more suited to soft surfaces. Larger tires can help, but there are limits -- they may not work in something like snow (which is why snowmobiles use tracks).
(0.75 psi snowmobile tows a 40 psi ATV)
Traction from tracks is generally better than wheels, but still depends on the depth and firmness of the tread being adequately matched to the terrain for best results.
(Much like a gear, a tire or track engages with the ground underneath it. Tracks simply put more "teeth" into the ground.)
Suspension is related to traction; without the ability to conform the tracks or wheels to the terrain, the traction doesn't come into play, and your motors will just spin ineffectively.
Building good suspension is much more complicated for a tracked vehicle than for a wheeled vehicle, but by no means impossible --
as shown in this iRobot video:
But, compare the climbing ability of tracked vehicles to the rocker-bogie system used by the mars rovers.
Steering is necessary for maneuverability, and this is where wheels can have a significant advantage. For skid steering, good traction works against you... and in uneven terrain, you might find yourself blocked from lateral movement.
There are some hybrid approaches -- 4 tracks, non-skid steering -- that approach a "best of both worlds" design: Mattracks and hagglunds, for example.
One very important balance to to all of these factors is maintainability. The complexity of tracks means that there are a lot of ways to break them; you're adding a lot of moving parts that will wear out independently, or be prone to dirt and debris causing trouble. Troubleshooting tracks will be more difficult than wheels, and tracks will be more difficult to service. So from a durability standpoint, if you can get away with wheels then use wheels.