I'm remodeling the downstairs bathroom. I have a raised ranch and on
the lower level at the foundation top we have a very wide windowsill,
about 8" or so. The walls will be porcelain tile up to about 5'. The
present sill is wood so I'm thinking of either a marble or granite piece
3/4" thick.

Is either of these materials not suitable for some reason? One better
than the other? Cost is not going to be a big factor, when spending
thousands of dollars already, forty bucks either way won't make much
difference.