Greetings Craig and welcome to the forum. Both sound interesting, but why not just use two sheets of 3/4" MDF laminated together with an edging of oak or any hardwood? Many of the guys here use this formula for their router table top. Mine is not MDF at this time but will be probably this summer or fall.
I will grant you the granite would be flat and smooth enough but really heavy. The phenol would not be thick enough for a miter slot or "T" tracks. I think I would look into the MDF with the hard wood edge similar to mine. (BTW mine is also heavy with the cabinets and all, but I can still turn it over to replace the castors which I will be doing this summer also.)
Wisdom: Where experience and knowledge combine and become one.
"We are all one decision away from Stupid!!"
"How often we sacrifice the permanent plans of God on the altar of immediate solutions"
I have a very good memory, just short is all.