Standard wheels on Sports and Sprint models (and GT400) are the 5" Azusa nylon with steel bearings. This wheel is very lightweight and is ideal on smooth runways. While this type of wheel is acceptable in most applications..... some operators prefer a wheel with more durability and a larger diameter for operations on less than ideal terrain.
The aluminum wheels are stronger, can take a higher side load, have better cosmetic appearance and naturally cost more than the black nylon wheels. The aluminum wheels have a finished look.
The spun wheels are polished and the cast wheels are bright and shiny. The 6" dia. wheel allows a larger selection of tire sizes. Larger diameters are better suited for operations on rough terrain and heavy loads.