Both of these facilities have their advantages and different situations where they are more appropriate. Spay-neuter vets are focused solely on combating pet overpopulation; they continue to fill a much-needed role of providing low-cost sterilization to pet parents who could not otherwise afford it. Most spay and neuter vets would even agree that if you have the means and ability, having your dog spayed or neutered at the veterinarian you have a continued relationship with is usually the best option for your dog.
Know someone waffling between a low-cost spay-neuter clinic and a local vet? Please consider sharing this information with them. In life, we all you know you get what you pay for, and the $30 spay is rarely any different.