How does wood hardness affect the sound of a TT on a wood TT platform?

Hi, I doubt it would have any effect on the sound. the question is what effect would it have on the isolation. The maple will be about 25% heavier for the same size. If your TT stand is on concrete and the stand is sturdy I would go with the walnut for aesthetic and durability reasons. Maple expands and contracts more than walnut with humidity changes so it is more likely to check with age. I can not think of any situation that the extra weight would be a sure benefit. If you have bouncy floors the only things that will help are tip toes or a well suspended table. I n that case a wall shelf would be the best way to go and again I do not think there would be a significant difference. I would go with the walnut, oiled if possible. Shame to lacquer such beautiful wood.

I think the first thing you have to ask yourself is whether your tt is sensitive to what it sits on.  My Linn was extremely variable depending on shelf material, while my Basis is not.  I don't know what your table sits on now, but try a different shelf material like glass or corian or whatever you have and see if it makes a significant difference sonically.  That said, I truly do not believe one can predict whether they will prefer one shelf material over another without trying it.  There are just too many variables to make a blanket statement that harder wood does X and softer does Y.  
And even if you could predict, that doesn't mean you would prefer it.  

Hardness of wood is related to stiffness which is useful in resisting bending forces, I.e., isolating against rotational forces. Hardness is also an important characteristic of the cones the wood board should be resting on.