There are a couple opinions on this. Biggest advantage is that the photographer has more direct control; not subject to interpretation by some printing lab. Because slides have to look great for professionals without resorting to printing tricks, most innovation effort has gone into making the slide emulsions better. Opposed to negatives, where most innovation is driven by making it cheaper. So, the slide emulsions are actually better at color saturation, accuracy, grain, speed, and sharpness than the equivalent negative materials. That being said, negatives aren't bad, particularly when the final result is a paper print or computer screen. (It seems to me) the intermediate processing steps here negate the advantages of slides, and negatives are more forgiving of errors.