It's much more complicated than just the porosity. See http://www.stonecaretechniques.com/6_m.htm
for a good discussion.
The main concern is absortion of liquids by the stone. Marble is more absorbent because the porosity is physically more interconnected. In granite its more "fractured" and random depending upon the makeup of the stone. They are pretty close actually, but it's the hardness that makes granite more desirable.
I don't want to argue the technicals, but to answer the OP's question, I say use whatever feels good, cleans well and passes inspection. Marble in the kitchen is a historical thing, until granite was cheap enough to mine, cut, polish, etc. for the mass market. Think of it this way, if you wanted a really heavy surface back then, you used a butcher block or some thick wooden bench. When that went out of style due to health concerns, stainless steel and stone were the alternatives to increase sanitation. Marble at one time was cheap and readily available.