When aluminum is oxidized (all the time, if not scratched) it has a really high impedance.
I went with (ply)wood. It's easy and cheap to make changes and that's a really big thing to me as my rig seems to be evolving all the time. With a proper varnish it's easy to keep clean too.
I didn't think about safety at all when making the decision. I think that it's enough to have all the metal parts grounded by the book and everything behind CFI.
An Ohm meter doesn't give you any readings, if you measure water. Only thing that can be tested whit it is the Ohm value of the ground connection. An insulation tester ("megger") is needed, if you want to evaluate safety of the build more preciously.
I like wood, purely because it's been easy to modify as my set up has evolved over time.
There's a health and safety aspect to that, having everything well situated, no hot and or electric components propped up on blocks or the like.
I agree with the sentiment that the choice shouldn't be a factor with grounding things, safe is not relying on what the stand is made of.