[Lil' Lavery]

It will depends on what you want your robot to ultimately do. A 2WD will grant pinpoint turning (although sometimes bordering on hard to control), but will typically also make it easier for other robots to rotate your robot. Depending on the style of a 2WD, it may also make it hard for you to travel up inclines or steps. A 4WD's turning ability is heavily dependent on the placement of its wheels. The wider and shorter the wheelbase, the better it will turn. A shorter wheelbase also helps prevent you from bottoming out (one of the largest issues with 4WDs and ramps), but a shorter wheelbase also means the robot is less stable in that direction, and thus easier to tip. A "co-planar" 6WD (all 6 wheels in contact with the ground at the same time) is essentially the same as a 4WD, but it helps prevent your robot from bottoming out and spreads out the weight distribution on the wheels. A 6WD with an offset wheel (so that the robot "rocks" back and forth between two sets of 4 wheels, the middle staying in contact with the ground) helps turning (by effective shortening the wheel base) but provides the stability of a longer wheelbase.
2 motors are simpler, lighter, and cheaper than 4 motors in a drive-train, but also provide half the power (assuming all 4 motors are the same). Drive power is typically a premium in FIRST bots, and especially considering the wide range and large quantity of motors available this year, I'd heavily suggest using 4 motors.
Seeing as you're a rookie team, I'd advise you to create a simple, yet fast and powerful, drive system. Consequently I'd advice you to use a 4WD with 4 "Small" (2.5") CIM motors.