What is the advantage of a four-wheel drive vs a six-wheel drive?

Four wheel drive tends to be lighter, and less complex.. with a six wheel drive (such as in the M35, M39, M913, etc. series military trucks), you usually have to have an interaxle differential lock. In normal operations, tandem axle trucks, such as dump trucks, tractor trailers, etc. only have one axle which drives them forward. When they need to, the driver can engage the differential lock to engage the second drive axle. On a 6x6 truck, such as the type described above, the transfer case would be used to engage the front axle.