marble or granite slab under floorstanders?

Definitely Granit. From a countertop store that hadles stone tops etc. Google it in your geographic area.
You might also want to consider a 3" or 4" thick maple butcher block sold here on Audiogon.
https://www.audiogon.com/stores/bba

Go to a counter top dealer...they have scrap pieces to fit your needs Cheaper than a audio dealer....The same product....

Google “granite surface plates” which are typically sold at machine shops and are dirt cheap compared to other retailers or kitchen counter shops. 

Better isolation, @Corian  ATH  composite. Granite alternative and much better sonic attributes, Very quiet. An far superior than either marble or granite in terms of "Ring"

It's also easier to work and REPAIR if needed.

Regards..

I’m sure either works well. Get scraps from a stone supplier and you’ll have to pay for fabrication. Shouldn’t be more than 100 per speaker.