If this sill is gonna be in a bathroom, I'd definitely go with granite.

That's because marble is really nothing more than metamorphed limestone, and limestone is readily etched by acids.

And, when you think about a bathroom, there's acid used all over the place:
- toilet bowl cleaners are typically either phosphoric acid or hydrochloric acid based
- general bathroom cleaners use phosphoric acid as the active ingredient because it cuts through soap scum without attacking chrome plating.

So, if it was my bathroom, I'd go with granite because it's less susceptable to damage caused by acids. Some good intentioned person is going to put a bottle of phosphoric acid based bathroom cleaner on that sill and cause some damage. If it were a granite sill, there'd be considerably less damage.