Both are good. Limestone is very good for inverts (craws and shrimp etc) as they feed in the rocks on items that have absorbed some of the nutrients from the limestone. That is good for shell growth of the inverts. Cost may be a factor. Granite will last longer (limestone will last a long time but not as long). The structure thread has some ideas and more info.

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=92463#Post92463