The types of materials that they are made from are important when considering your use of the tub. The most common material used in free standing tubs is acrylic. Acrylic bathtubs are great because they are non-porous, non-staining, and easy to clean as well as being lightweight. Acrylic is a very popular choice by manufacturers because it is relatively inexpensive and very flexible and becomes rigid when it is formed, cast, and reinforced with fiberglass. While acrylic can scratch it is easily buffed out and resists staining very well. My only complaint with acrylic tubs is that they aren't great at retaining heat, which is kind of an important part to taking a bath.
Cast iron tubs have been around the longest and are still popular today; they are very sturdy and retain heat well. However they are very heavy and that can present some problems if they are being installed over a joist system like a second floor or a house that has a basement or crawl space. In those cases additional support maybe required to support the tub when it's filled with water and an occupant. The other draw back to cast iron is that the enamel layer that is applied over the cast iron can chip and stain but it is able to be touched up or refinished.
Metal freestanding tubs are popular for their aesthetics. A sleek stainless steel tub gives a very modern and clean look to a bathroom and has wonderful reflective properties that can make the design of the bathroom really pop. A classic hammered copper tub can give you the rustic/traditional look and bring in some old world charm. Metal tubs present the problems you would expect with a metal product. Cleaning them typically requires a special cleaner and dents and dings can happen. Even though a majority of these tubs are double walled they do not retain heat very well.
Wooden tubs are the perfect fit for if you are looking for the cabin or rustic look. They come in all types of different wood species and colors. They are very easy to take care of, typically with just a mild soap and water. The trick with wood is that they are meant to be used regularly and require certain moisture levels that prevent the wood from contracting and or cracking. So if you want a freestanding wooden tub but don't always take baths you'll want to "wet it down" periodically. They maintain heat well as they are usually pretty thick and dents and dings are usually part of their character.
Stone bathtubs are great looking tubs that retain heat very well and can lend to any design aesthetic. They are made from a wide variety of materials like granite, marble, travertine, and even volcanic limestone. Stone tubs are my personal favorite because the natural beauty of stone is untouchable by other materials and their versatility in shape, size, and finish are outstanding. They are heavier and may require additional structural work like the cast iron tub, but hold temperature extremely well. When it comes to the volcanic limestone you'll find in tubs from Victoria + Albert it is actually volcanic limestone combined with high performance resins to make their Englishcast products. Not only do the hold temperature extremely well they are also typically warm to the touch too!
With all of the different types of freestanding tubs on the market it can get confusing trying to pick the best tub for your application and budget. One thing is for certain, the plumbing rough-in for the drains must be spot on and a tub filler will be required and typically a floor mount is preferred. These bathtubs can really add to any bathroom project and can alleviate the need for bulky framing and tile work. Choose the style and material that best fits your needs and you'll be happy for years to come.