H votes to match the wood floor stain. I'm not sold with it 100%. I wonder I we should go darker? Or maybe medium brown or medium reddish brown? See, I'm really not sure. LOL.
Lurker her but I would find a stain that pulls out the browns in the fireplace stone. I can't tell from the pic how much red there in the stone is but I would probably lean more medium brown.
I think the yellowish hue of the floor on the mantle would just make pull the yellow of the stone to much. Plus the darker color would give the mantle some additional presence.
I'm voting on your side. A white mantle sticks out like a sore thumb around all of the stone. I'd try a medium brown. If that didn't look right you can always go to a dark brown.