Given the choice, I'd prefer a small number of high-quality, color pieces over a lot of simpler, black-and-white sketches. I know that's not always easy to manage, though, especially for bestiaries and other image-heavy products, and I understand if companies feel sketches are more appropriate for a given product. (I can't speak for others, but art is practically never a selling point for me. I buy based on the flavor and rules content I expect to get, not any art associated with it.)
Consistency in a given product is definitely important.