There are so many factors- hard to just answer that question. Stainless steel will generally outlast the truck chassis; but that is a large investment upfront. I’ve heard of a stainless tank lasting for 2 truck chassis life’s. Steel is the cheapest option, but the heaviest and most prone to pitting/rusting problems. If you have a hard time finding (or paying) CDL drivers but need a large capacity tank then aluminum is the best option. All of the options have some definitive pros and cons. Do you operate in a region that uses salt brine in the winter? If you do have a problem with the tank, steel is definitely the easiest material to repair or work on.