From an engineering point of view, it comes down to cost.
Distortion is not an issue if you have qualified welders using the correct weld procedures. You could make the whole thing out of aluminium wide flange if you wanted.
Space becomes an issue with wood as you get larger and larger but realistically as the stands are generally in compression wood is more than strong enough.
Intuitavely, I would think that wood is cheaper than steel and construction costs tend to be less but the only way to be sure would be to get a quote from a shop.
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