I think white marble always stands out more, so I guess it depends on whether you want your fireplace to be a feature within the room. Both sound elegant though, and I'm sure either would work with grey and oak!

We have a house in Spain with marble floors and have this issue in several rooms- we noticed the main areas after about 6 months but have had new areas emerge even 15 years later. We’ve had several specialists to look at it over the years and had explanation of oxidising, damp coming through, being laid on sand, sand walked on them, sea air, wrong cleaning materials ...We’ve had the floors specialist cleaned, ground and polished many times and it’s still there. I know some neighbours just replaced them with a less porous stone. Could you ask the marble supplier as I think you do need to avoid it pre installation not rely on being able to clean it afterwards.

Do you have any photos of the room? What style & colour of units and worktop? The glass will look great with more modern, slab styles; the marble may work better with shaker units. There are so many marble-style splashbacks, though, and some are quite heavily patterned and stark. Softer, whiter marble-effect quartzes can look stunning - especially if the worktop matches!

If you want to strip the paint off, have a look at a company called Strippers paint removers. They have a very good website, I have used two of their products to strip paint off different surfaces with really good results. It is a bit of a messy process and may take a few days but it will be worth it.