Aluminum may not be the best being a welder myself aluminum can crack (around the welds) under a lot of vibration I'm not here to say steel is better than aluminum but if it were me putting a back rack in my truck I would use steel
Aluminum pros: lightweight
Durable
Clean finish
Good conductor (for putting lights on)
Cons
Soft
Not as easy to repair if bent cracked or broken
Difficult to weld ( without ample experience )
Steel pros
Strong
Extremely durable
Many types to meet your liking
Easy to repair
Easy to weld
Can be welded with arc (stick) welding a stronger bond
Cheaper
Quicker weld fab time
Cons
Rust over time ( compared to aluminum )
Heavy
May cause vehicle stress over time if not put-on properly
There are many more pros and cons to both do some research on what would be best to your liking hope I helped