What is the cost difference between marble flooring and granite flooring?

This depends which specific marble you are comparing vs. which granite. Some granites are less expensive than some marbles and some are more.

In terms of labor, the installation costs should be the same.

In terms of durability, usually granite is harder/will last longer. Marble is more fragile and has fault lines and hence it is more likely to crack over time. This is especially true if you get a cheap marble or buy it at a big box store (they tend to add a lot of epoxy/filler and you can't see this unless you open the boxes and turn over the tile.