Don't forget to compare file formats:
 
The model from Vanishing Point should include various formats: Poser, Vue, 3D Studio Max, Lightwave, etc. Though please check your 3D World CD to make sure the 3D World people put all the models on the CD.
From the looks of the description page at DAZ, their model only includes the Poser version, though it's probably safe to assume that it also works in DAZ|Studio.
 
If all other factors are comparable (turret length, hatches, treads, size, etc), then the next comparison would have to be price:
The tank from Vanishing Point is free with the purchase of 3D World magazine.
The tank at DAZ is $29.95, though it may be cheaper for Platinum Club members.

VanishingPoint... Advanced 3D Modeling Solutions