Well, I have to say that currently the version that plays on Mars follows a different concept that the invasion scenario on Earth.
It is rather planned as small "just-for-fun/beat-the-Aliens" alternative to the big, slow Earth invasion version.
I´ll release two versions of the invasion scenario: one starting in the first tech era with a weak start for the XCom so that you have to build up first before you can do much against the Aliens. This will be quite similar to a standard civ game.
The other version will have an "advanced start" at the beginning of the second era - more bases, troops etc. to get into action sooner. This version will have less turns.
The version playing on Mars has a quite small map 100x100 instead of the 140x140 earth map. Furthermore it is a flat map, representing just a part of the mars. It has no polar icecaps therefore, and there´s no water.
It start at the beginning of the 4rth era, with most of the units available, has only two sides (XCOM/Aliens), and will be even shorter. Irrigation will not be possible, and you can´t build much bases (and there isn´t enough time to do so), it is much more warfare oriented.
OTOH there will be some surpises, eg. an Alien fortress complex in a part of the map, consisting of several cities protected by a special defence system (impassable terrain).