Good question, Emilio. I have never seen the answer or any discussion on the question. Regardless, they are generally used for different purposes, depending, so the question may be moot. Both move with temperature and with loading, and cast iron often moves with time. Granite surface plates wear less and are much quicker and easier to calibrate than cast iron plates, unless the iron ones are ground. Smaller cast iron plates are often used for machine repair and calibration by taking the CI plate to the machine. Granite plates usually have the work taken to them. The advantage of CI plates is that they are much easier to calibrate yourself, if you have the scraping skills and an accurate master plate, usually granite. CI plates are also used for less than high precision work in shops not in need of it, for layout and less critical measuring and testing work. The CI plate can be used on rougher work, and then re-calibrated as needed in the shop, without sending it out, but again also requiring a good master plate. As purchased, ground CI plates are typically less accurate than scraped ones, and require regrinding to return them to original calibration. More than you asked for, and no real answer, but perhaps that helps...Do temp fluctuations adversely affect cast iron surface plates more so than granite?
The precision surface of the Busch plate is only 3/4" thick, but the complete plate is 6" thick and weighs 180 lbs. Here's a link to their catalog describing surface plates:There is a guy in Iowa selling two cast iron plates both 18" x 24" for an offer. I know one is a J.C. Busch Co. .75" thick. Think I might have to pass it up.