We should practice our terraforming skills on Earth first. When we are able to improve the atmosphere and climate of Earth (easy mode) we can try our skills on a remote and hostile planet like Mars (hard mode).

I'd consider Earth to be hard mode too, since it's already in production and riddled with irrational users who run their entire business on the memory leaks you want to patch. ;-)

Everyone is always seduced by the new — the new idea, the rewrite. But often what one finds is that as development goes deeper, the same bugs resurface, the same challenges. Our society is full of grand, ambitious, “let’s do it right from scratch” plans — Le Corbusier, etc — and they often fail. The blank page is seductive, but it begins the to fall apart with the first mark of the pen.

Conversely, the unpredictable nature of our first big mistake as we transition Terraforming out of science fiction and into text books argues for having a backup planet in place before we try it here.

We can and should do both simultaneously. There are lots of humans.