It depends on what you want and how much you want to spend.
Fiberglass tubs can be really good, but a marble tub is in a whole different category. For the price of a fiberglass tub jacuzzi starting at $700.00 you can start to think about a marble tub.
Lowes has a really good fiberglass tub, nice and deep and long enough for a big guy, and older folks really like its sides...a design of a much more expensive tub. Get a fiberglass and put marble tile around the top of it...you will get the best value for money that way, and done right, it will look great...stick to neutral colors...with a layer of a tile with some detail such as a raised impression for the Roman bath look.