Tom Lehmann, of the American Institute of Baking, once reported that they conducted tests using the same dough formulation but different forms of yeast: fresh yeast, active dry yeast (ADY) and instant dry yeast (IDY). They could not detect any differences in the final crusts.
Professionals use all three forms of yeast. The selection of what kind of yeast is usually based on cost, habit and convenience. High-volume pizza operators often select fresh yeast because it apparently is the cheapest on a per-unit basis. ADY, which came into being after World War II, gradually replaced fresh yeast because it was dry and had a much longer shelf life, especially if refrigerated. Fresh yeast deteriorates very quickly and must be used promptly. IDY was invented in the 1970s and, because of its smaller particle size and shape and fewer dead cells, has the advantage of not requiring rehydrating in warm water as is required in the case of ADY. The IDY can simply be added to the flour and other dry ingredients. I would say that today IDY is form of yeast that is most commonly recommended. However, several of our members say that they feel that they get better results using ADY. With ADY, however, it is important to rehydrate it in warm water (around 105 degrees F) for about 10-15 minutes. Only a small amount of the formula water should be used to rehydate the ADY; otherwise the dough may ferment too quickly. With ADY, the rehydration water temperature is critical and can lead to poor performance if not carefully monitored. Most failures using ADY are due to incorrect water temperature.
For additional differences, see the forum's Pizza Glosssary at http://www.pizzamaking.com/pizza_glossary.html