Water Pump V-Belt vs Serpentine Belt

Looking at buying a new water pump for my '78 /6.
Searching from my year/make/model, many sites list multiple pumps and offer a less expensive pump which states it is for a serpentine system. These sites offer both Serpentine and v-belt pumps for the same year /6's. There is a weight difference of 4 oz. between the two.
Is there really a difference between the two?
Is it the impeller size that is different between the two?
Is it the impeller shaft length?
Thanks,