Both products are well suited for a shower application. Porcelain will likely be less expensive than Travertine. Porcelain will have less of a variation in its appearance than a natural marble product such as travertine. Notice the beautiful variation in colour of the natural product. A porcelain will have one solid colour through out the tile.

Actually, with digital processes, porcelain tiles can have almost as much shade variation as natural stone. Look at the Regis Series by Cerdomus. Porcelain can be a more easily maintained product for use in wet applications.