Steel v aluminium comes up in two areas of interest to me, astronomy and cycling. Further, many of issues around the two metals are relevant to both hobbies. So heres goes:
Weight for weight steel and aluminium have close to same strength. I know this sounds wrong because of how they used but never the less it is true. What causes the anomaly is that many things made from steel don't need to be as strong as they are, so when they are made from aluminium they are made only as strong as required, thus using less metal and saving weight.
In cycling aluminium frames are notorious for transmitting vibration, where steel is better at dampening them. I cannot see why this would be any different in tripods.
An important attribute of aluminium over steel that everyone knows is, it is less susceptible to corrosion.
As for which to choose, I think it probably makes little difference as they weight the same, therefore as each as strong as each other.
So which ever you choose I hope you have clear skies to use it.
