There's nothing like a natural stone. If you go that way, granite is the most durable. Travertine will require more maintenance, depending on how you want to keep it, as lots more things will either etch or stain it...but, it can look lovely. You'd probably want to avoid highly polished stone for the floor - it's just very slippery unless you choose a small tile with lots of grout joints. Granite, you might want to consider flamed if you choose it for the pan, but that may not be the look you're trying to achieve, but it gives the granite reasonable grip.
From a maintenance viewpoint, porcelain is the least expensive, easiest to keep looking the same. Some of the travertine look porcelains are quite good, but will never have the variances of the natural thing.
The quality of your water, the types of soaps you choose along with the shampoos, etc., will all interact with the travertine giving it an even more unique look, but it will change and likely become less like it was when newly installed. Some love that, some hate it.
So, you have to decide what aspects are most important to you, and what look you're trying to achieve.