Is granite more porous than marble?

Yes, granite is more porous then marble. The pores in granite are generally much larger than marble and therefore will absorb more water. For this reason it is absolutely necessary to apply an impregnating sealer to granite.

FYI Most people consider granite to be a very strong and chemically resistance stone. This is true, but do not confuse acid resistance with absorbency. Granite is generally much more absorbent than marble.