Choosing a steel axle starts with determining what type will work best with your vehicle as well as how you intend to use the vehicle on a regular basis. Some steel axle designs are intended specifically for off-road vehicles, while others are intended for road use; some are drive axles, while others are dead axles that support the weight of a vehicle. On bicycles, this component may be solid steel, and on motorcycles, it may be a hollow sleeve. Determine what size axle will work best for your vehicle first, then consider other options based on how you intend to use the vehicle.
Steel is often the best choice for an axle because it is resistant to impact damage, easy to weld should the need arise, and generally affordable. A steel axle can, however, begin to rust or incur damage from impacts that can require repair or replacement. If you are considering purchasing a used steel axle, you will need to do a very careful inspection of the unit to find out if the rust is extensive enough that holes have formed or the stability of the axle itself has been compromised. Any cracks or dents will need to be addressed as well; a cracked axle housing can be welded in some cases, but this could be indicative of a bigger problem and the axle may not be very reliable as a result.
Sometimes it is a good idea to upgrade a steel axle for specific purposes. Off-road drivers, for example, often upgrade to larger drive axles that can transfer more torque to the wheels, or simply larger axles that can withstand the rigors of off-road driving. You will need to be sure the steel axle you are considering will fit your vehicle, and you will also need to take note of what upgrades or modifications will be necessary to accommodate the new axle. Consulting an auto mechanic is always a good idea, as is contacting the manufacturer both of the vehicle and of the axle to get the best information about upgrading.
Specialty axles such as portal axles will not work with all vehicles, and they can be cost-prohibitive, but they may be the best choice for some applications. Choose the axle that will fit your budget, and make sure you know what other components will need to be upgraded along with the axle. Drive axle replacement may require the replacement of the rear differential, or even the driveshaft itself, so take note of the compatibility of such components before replacing the axle.