As with ZIS-150/ZIL-164, the model of ZIl-151 (ZIS-151) can be converted into ZIL-157.
But considering complexity and cost of mold design, ZIL-157 alone would be just fine since it was by far more popular vehicle (36 years of production) in almost every conflict all over the Globe.
Ultimately, I would prefer ZIL-131 since I drove one myself.
On contrary to 4x2, 6x6 trucks also the platforms for numerous weapon systems like BM-13, BM-21, pontoon carriers, missile transporters etc...