A:  If you are comparing the two coatings for cost/gal, then the solids volume solids is important. If you are trying to minimize your VOC emissions, then you need to calculate how much VOC would go into the air if you were to use the same volume of solids to cover a particular job.  Clearly, a coating with high solids content goes further than a coating with low volume solids. Therefore, you cannot simply compare the VOC values of each coating.  You must perform the calculation.  

 

With regard to performance you would need to evaluate both coatings. There is no formula or rule of thumb that say that a high solids coating out performs a low solids coating when both coatings are applied at the same dry film thickness.  If you accept that higher solids coatings can generally be applied thicker than lower solids coatings, you might argue that the corrosion preventive performance of the thicker coating might be better, but a similar argument cannot be made for chemical resistance, sunlight exposure, etc.