Cultured marble is definitely better as your bathroom has a different look with the marble. But since the walls on the four sides will have glazed tiles you can also go in for tiles on the four walls and matching dull tiles on the floor. Nowadays i think tile is the fashion.
Tile is best suited as it is avialable is many new designs, shapes and colors. Cost is economical. Flooring can be easily mathched with the chosen tile. Various design patterns can be created. Mix and match with different tiles to create unique design is possible.
Marble is beautiful, but it is also very soft as far as stones go. It is very difficult to keep marble polished on countertops and floors, and it will etch if in contact with acids. Ceramic/porcelain tile usually is not as beautiful initially, but is much easier to maintain.
I would have to say cultured marble. Marble by far is a better product. In the bathroom you have to think about the water that the product will come in contact with. Marble out lasts (in my opinion) any product available.