I could be wrong, but I think a granite plate is better for an inspection plate, due to its stability and hardness.
Cast iron may be called for if you want to lap something on the plate, because the cast iron will take a 'charge' of abrasive, ie., some will embed in the surface making it a good lap because instead of the abrasive tumbling around between two surfaces, it will be held by the cast iron, and do the cutting mostly on the other part.
A good example might be lapping a granite plate flat: rather than use three pieces of granite against each other to try recondition them all flat (it takes 3 flats, lapped against each other to guarantee that a flat plane is produced instead of a spherical surface), it would be better to use 2 cast iron flats and the 1 granite. The cast iron laps can be grooved to improve lapping performance. They can also be restored to near perfect flatness relatively quickly on a lathe or mill, without fear of polluting the machine with abrasive grit forever after.
I've also done some lapping of various pieces on my granite plate, but not directly on the plate. Rather, I use some self-adhesive disks (commonly found in body shop supply houses) and stick one of those onto the granite and lap away. These abrasives are easily replaced as the abrasive wears out, and it is easy to stick a few different grits down onto a 24 x 16 granite and make a nice lapping station.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)