I chose to have my CM shower surround and base done with a matte finish and I love it! Just wanted to avoid that 'tacky' cultured marble look from the '80's. We just had another base custom made for our Master bath, and when I asked for the matte finish again, he said he couldn't do it, cuz of the way this one was made over the last one (Don't quite understand the technical details), or I would lose the non-slip finish. The way they get the matte finish is to sand down the marble. Anyhow, just took a peek at my new base (Most of it is still protected with cardboard as we haven't tiled yet), and the sill portion looks beautiful, not really glossy, not really matte, if I had to describe the finish, sort of like how ceilings are done now.

If you are having it custom made (and my guy can make it any colour/pattern I can think of--I had my base matched to my wall colour!), then I would take a look at the show room and see what they look like in person, quite a change from the CM of old!