Presumably if someone has the technology to come here, and they want resources, then it might be advantageous to go to Mars (lots of land and even water in polar ice). It's much easier to come and go there than here on Earth.
Plus, there are fewer microbes and stuff that might be hazardous to their biology, as H.G. Wells's creations discovered. And even oxygen is pretty [link|http://www.eurekalert.org/pub_releases/2004-04/nsf-oha042004.php|hazardous to living things that haven't found ways to adjust to its presence].
Just some idle musings...
Cheers,
Scott.