I realize that the tile has the look and feel of money, but if you were showering 1-2 times a day, which one would you choose for ease of care and feeding?
Even if you have to put it in in several pieces (base, two side...), from a maintenance point, it's easier to put a few beads of caulk than maintaining grouted walls. However, prep the walls in a water tight manner! The oldest one I have in use is 15 years old and it still looks and works great(3 piece unit).Good luck!
After removing the existing shower or tub, and removing the wall board, it's easy to drop in a new tub or shower pan,
Install new backer board,
And then glue the marble panels up, using Silicone at the joints.
The panels can be poured to cover the hole left in the room by the removal of the drywall.