I hope this helps!
Go with the flooring materials. There are certain requirements that has to be met, and granite for flooring is hardly a good option.
Ii am not even talking about the cost. But at $40 psf (a giveaway price), you would need a check just for materials for $100,000. And it would be very cheap granite.
I do not believe you would increase value. Probably, the opposite. People would say that they need to replace it, and replacing it would be costly, plus then doing "normal" floor is also an expense, so they would most probably discount the price to do it.
Please feel free to contact me if you have any more questions. Good luck.
Leaving aside concerns about practicality. I think it will be high maintenance floor, that you need to keep sealed, as it is porous.