Supposedly slides are far superior to digital projection, and I could very well be persuaded to agree -- but at the same time, digital projection is kind of very crap these days at the low end, just like any other digital display technology.
Good digital projection beats the hell out of film, which is why the movie business has moved to digital*. Of course that good digital projection is expensive, but as a practical matter it takes a high quality, dedicated projection environment (e.g. a serious home theater room) before the quality of the projection is the limiting factor in quality.
*No, it's not just a cost reduction thing. In some of the tests where Hollywood was trying to figure out if digital projection was ready, film was literally booed off the screen in back-to-back comparisons. The qualitative difference, even with new prints, was that big.