Races
Marathon: Invasion includes 6 races:

Each race has a different armor/resource cost modifier, and have distinct advantages over each other race.

The UESG, the human government which originally commissioned the Marathon, has relatively large numbers, and ships a bit on the weaker side. They have just received Durandal's message concerning the Pfhor invasion, and have set of to eliminate the Pfhor he spoke of.
Armor/Resource Cost ratio: 1.00

The classic enemies of the Marathon Sage, the Pfhor have come out this time on the defensive, attempting to stall the UESG advance and hopefully destroy the possible menace. Previously the strongest race in the quadrant, they have been offset by the reappearance of the Jjaro and the rejuvenation of the S'pht'Kr.
Armor/Resource Cost ratio: 1.50

In the last chapters of Lh'owon's history, the S'pht'Kr returned to assist Durandal in destroying the Pfhor, and they hate the Pfhor with a vengeance for enslaving their kinsfolk. They will assist the UESG in any way possible in order to destroy the Pfhor.
Armor/Resource Cost ratio: 1.75

Fragmented Pfhor records gave Durandal knowledge of the Nar, another existent race in the quadrant. While generally of little influence, their homeworld lies almost directly inbetween that of the Pfhor and Earth, pulling them into the giant conquest. Torn between allying with their mortal enemies, or those who from previous experience they would believe crushed at the Pfhor's first opportunity, they have attempted to strike out on their own.
Armor/Resource Cost ratio: 0.75

The nomads of the region, the Nebulons, are also present, while hardly a "faction" of any sort. While weak and relatively pitiful in the eyes of the other races, they hold sway because of their massive numbers if nothing else, and the small but present importance of their trade to the region.
Armor/Resource Cost ratio: 0.35

The Pfhor enjoy their exalted status because of one race: the Jjaro. As the most tecnologically advanced race the galaxy has ever known, the technology they left behind when they sojourned from our galaxy was easily used to enhance those that found it: the Pfhor. But in their millions of years of absence, they have developed some new tricks...
Armor/Resource Cost ratio: 2.50