magnesium is sort of a more expensive version of al.

In that it's one electron from it (like Silicon)? Magnesium is 2.2x stronger, 1.08x harder, and 2.05x more costly, 0.65x as thermally conductive and, 0.64x as dense. Think I'm missing your similarity metric

"nl" already brought few points. As a practical test: take a piece made of cast magnesium (alloy) or cast aluminum (alloy). It'd be hard to easily tell each other apart, save for using a weak acid. Their strength is similar (esp. when alloyed, still worse off for the aluminum) but magnesium is non-trivially lighter. Here, a random quote [0]

Magnesium is also better at casting components with thinner walls and tighter tolerances than aluminum. However, even with the many advantages of magnesium, aluminum remains a less expensive alternative for die casting.

[0]: https://diecasting.com/blog/the-difference-between-aluminum-...

Firstly those characteristics are quite similar for different metals.

Secondly Aluminium is normally alloyed with other metals bringing the two even closer.

Finally it's a light, strong metal and used in many similar industrial products as Aluminium.

What do you do that the differences between magnesium and aluminum are noteworthy in casual conversation? Machining?

You just reminded me that I used to have a camera with an magnesium body. That thing was a delightfully tough beast.

It is also very dangerous to machine. https://www.china-machining.com/blog/cnc-machining-magnesium...