From what I know there is no difference between toughness of army/marine/guard units. The difference is in what TAC areas they cover. You can cover almost all of the tac areas with army units and other starting stuff so there is little need for them for most things. Each ground unit covers a number of TAC areas, from 1 to 8 areas. Each unit has a rating that relates to the TAC areas. If you want to see what an army can do you will need an enemy to practice on. However if you like I would be happy to post some old battles so you can see what to expect.
Imperial Army Air Force has the following
Air Def 10%
Air Support 20%
Air to Air 70%
The Imperial Marine Air Force is
Air Def 20%
Air Support 40%
Air to Air 40%
Other that a fortress all units total up to 100%. You can see the difference between the two. Both units will provide tac ratings in all three areas, but they will be different given the same tech. If you have all ground units at your disposal you can tailor your force based on enemy strengths and weaknesses. Ground techs will say what areas they are good for.
Ferret Excavator Tank: The Ferret is a tunneling tank designed exclusively to provide heavy armored firepower
against subterranean enemy targets. Armed with high explosive shells and a massive drilling rig attached to its front, the Excavator can make impressive progress against any dug-in position. In extremis, Ferrets are used to plow through buildings, walls, fortified bunkers or just about any hardened position. After blowing through the outer
defenses, the Excavator lets loose with a hail of gunfire and often leads the charge for more conventional infantry forces. The presence of Ferrets can strike fear in the hearts (assuming they have any) of an unprepared enemy.
Classification: Ground Combat
This Item cannot be manufactured, and is either a ground combat upgrade or a prerequisite for other technologies
Prerequisite Technologies: Light Tank Armor Strength: Adequate 
Broken Terrain Strength: Adequate 
Subterranean Terrain Strength: Adequate