The advantage of being on Mars is that as your population expands you mostly need to just dig more tunnels for them to live in. With a space station you have t

The advantage of being on Mars is that as your population expands you mostly need to just dig more tunnels for them to live in. With a space station you have to build more space stations or somehow add on to your existing one, which is very difficult with artificial gravity setups. The explosive failure modes of a space station are less of a concern on an underground Mars colony as well. But this of course raises the question as to why you would do either instead of staying on Earth. Elon Musk talks a lot about a multi-planetary species, but it's extremely unclear how long it would take for a Mars colony to become self sufficient. And anything you can do underground on Mars you can do underground on Earth but with fewer catastrophic failure cases. The only threats a Mars colony protects against are a dinosaur killing asteroid strike and a completely runaway greenhouse effect turning Earth into a second Venus. Even worse, if Elon were serious about the Mars colony he should have multiple fully self-sufficient test colonies on Earth vetting out the technologies now, but if he had that it negates the need for the Mars colonies.