Mortar and Pestle: Granite vs Porcelain
Neither.

The traditional instrument for making guacamole, and other Mexican mashed and ground preparations, is the mocaljete, which is similar to a mortar &amp; pestle, but made with a shorter pestle and out of black basalt, a rough volcanic stone:



A regular mortar and pestle out of ceramic or granite will not work for a fat, squishy fruit like avocado; it will just squish around and out of the bowl.  You need the rough, pitted texture of the volcanic stone to "grab" the avocado pulp.

Personally, I find that the ideal tool for making guacamole or avocado salsas is actually a regular mixing bowl and a heavy pastry blender:



... but even a large metal serving fork will work fairly well.  Better than a regular mortar and pestle will.