Typically, avoid steel at all costs. It might be cheaper, but it is significantly heavier. Most higher level teams resort to different forms of bracing with the help of geometry and physics. Chances are you can reinforce aluminum to be just as strong, if not stronger than steel in only certain areas .
There are few applications where you need an entire piece of metal to be as strong as steel (and I mean very few). The one notable place I used steel in TT was for my anti-tips because box bolts/couplers simply could not fit in the area I needed. Notice how high of a load an anti-tip withheld in TT. The entire mass of a gigantic robot, 11 cubes (5kg+), and high amounts of momentum.
The key here is being able to identify the weak points and use things such as box bolts and triangle bracing to adjust those areas.