Greetings Kimberly:
Sandy soils are coarse-textured, contain less organic matter, and typically have lower cation exchange and water holding capacities and base saturation.
Loam soils, with more clay and silt, typically have higher organic matter contents and water holding capacity, and greater cation exchange capacities and base saturation.
Based on my experience with white pine in Illinois, both loam- and sandy-textured soils are preferred for white pine vigor and growth. Unfortunately here in Illinois, too many people establish white pine on fine-textured soils which results in huge internal soil drainage issues for the white pine root systems.
Are the soils you speak of truly "sand" vs. "loam"? If anything, I would think drainage is the issue and not fertility; otherwise the loam site should fair better. Quality of planting stock, planting depth, and the handling of planting stock are other factors to consider.