Fiberglass tub is standard equipment.
I would look for a large Garden tub, maybe add some jets.
To answer your question go with the fiber glass. to invest all that money in marble someone might not like.
Below is a link to some nice marble / stone tubs. starting at 2K they go up in price quick
Check http://www.EubankContracting.com for examples
ARC International Onyx manufactures and distributes Stone Bathtubs in various colors, designs and shapes. Each ARC International's stone bathtub is custom designed so to blend with the architectural specifications of every client. If you donâ€™t see what you want there, let us work with you to get the Onyx bathtub or other stone bathtubs that fits your needs with the look you want.
Bathtub hand carved into a solid mass of natural stone has and always will be within the reach of only a privileged few. The selection of a suitable block of stone requires the highest degree of professional experiences, and the characteristics of each stone, either Onyx or onyx block guide the ultimate design and shaping of the Onyx or onyx bathtub. The pure joy of bathing inside the heated stone is an experience never to be forgotten.
ARC International offer bath tubs with rich luster of polished, unmistakable and long-lasting Onyx. The Onyx bath tubs baths are intricately carved and stand out with the striking patterns, sharp details and high polish.
Btw, parts of my area in Westchester, NY are very high end. I haven't run into marble tubs yet - might happen, but not so far.