Good morning Jane. Been installing custom showers with both marble and tiled walls for a lttle over 32 years. Both perform well, so my suggestions are about the subtle differences almost to the point of splitting hairs LOL. Given a choice , I'd go with a full bodied, rectified porcelain. Marble is for the most part a naturally porous material, and if water gets to the unsealed surfaces will eventually wick in, leaving ( in the case of say..carrara) a slightly yellowish stain ( from the iron) Most marbles are sealed well on the face,but not well on the edges and back. The key is to be dilligent on sealing the grout routinely, not allowing water to permeate through the grout, and on to the unsealed edges. Full body porcelains have come a long way in 20 years, offer many selections, are price competitive, and are highly durable. And they do not stain out. All this being said, marble showers I have done many years ago still look good today. Hope this helps. God luck!