Kind of a loaded question.
Strength to weight a heavy skid plate in aluminum 6061 is going to be much stronger than a heavy skid in mild steel of the same weight/design still have good flex but is also more expensive.
There are 7 main aluminum alloys, they all have different properties, which are you looking at?
What kind of steel are you talking about? If it's just mild steel its got some give, which is good because if you hit something hard its going to take some of that impact, but if your talking about other steels they have other strengths and weaknesses vs mild.
There is also potential for reinforcement (think ribbing) that could help the plate take an impact better. But again, having the plate deform is a lot better than bending your frame, plus transferring more force to the driver.
There are design questions, what angle is the skid plate at? Effective material thickness changes with impact obliquity. Stamped ribs can add a lot of rigidity for no weight penalty. Welded reinforcements could also add a lot of strength, but there is the weight penalty and additional cost of fabrication.
Like
@zombiekiller mentioned, skid plates are there to protect stuff, they are sacrificial. I would much rather mangle a skid than tweak the mounts (frame).
Did you have some specifically you were looking at?