What are the relative merits of aluminium or quality steel? I'm talkimg
with reference to touring bikes

If you are going around the world, then get steel as you can get them
repaired everywhere.

You can probably also get a new frame shipped to you, which may be
better than having the local blacksmith patch it up.

Have you considered bamboo?

A small advantage of my Al frame is that my handlebar bell/compass works.