Although the three factions in Supreme Commander appear quite similar on the surface, there are some differences between them that definitely result in a different gameplay experience, especially when you build up the tech tree. This chapter is going to go over some of the aspects that makes each faction unique
The UEF is probably the first faction that you'll encounter when you play Supreme Commander's campaign. This zealous faction wants to restore the rule of law to the galaxy and isn't above destroying the opposing factions to do so. It plays an unsubtle game of sheer power, with few options for stealth.
T3 Heavy Gunship: An extremely powerful unit for the cost associated with it, this Tech 3 air unit is much more powerful, both offensively and defensively, than the T2 regular gunship. Although they will still be sliced up by SAM installations, they can shrug off flak cannon fire, for the most part, at least long enough to knock out that nuke installation that your opponent is constructing.
Fatboy Experimental Mobile Factory: The mobile factory packs short-range artillery firepower as its main offensive capability, and it's quite capable of chewing through ground-based units with ease when they get too close for comfort. It's also capable of acting as a mobile ground factory, allowing it to create more tanks and engineers for you as it rolls towards the enemy base. It can't create T3 units, though, except for T3 engineers. The units it can create, however, get created in around half the time as it would take a T3 land factory to make them, at a commensurately greater resource cost over time. The factory cannot construct units and move at the same time, however.
In addition to its firepower, the Fatboy has an excellent 25,000 point shield, allowing it to stave off a good amount of damage before it starts getting hit itself. It can also repair and refuel aircraft (you can even park a couple of strategic bombers on it and transport them towards an enemy base under its shields and immune to anti-aircraft fire), float onto bodies of water, fire torpedoes, and write NASCAR-themed Harlequin romance novels. All in all, it's one of the most well-rounded units in the game. It does have a constant 500 point energy drain, though.
Atlantis Experimental Submersible Aircraft Carrier: The Atlantis can be a devastating tool against enemy bases when you have it on your side of a fight. It can carry an extremely large number of planes in its berth, and since it itself is radar and sonor-shielded, you can submerge the carrier, slide it along the water's bottom up to an enemy base, then surprise them with an aerial bombardment that they had no way to see coming. The ship does need to be raised to the surface of the water to disembark its planes or repair and refuel them, but if you drop it back down underwater, it acts as a formidable underwater torpedo launcher, and will be difficult for your foes to take out unless they have a large number of submarines or torpedo planes.
The aircraft carrier is one of the cheapest experimental units in the game, but that's mostly because it's only useful if you manage to build a huge number of airplanes to fill it up with, which will take a good amount of time. An enemy that's shielded up and which has a large number of SAMs or air superiority fighters will be able to destroy your forces as they arrive, although if you're lucky, most of the defenses will be situated between the two bases; if you manage to sneak around to a location that's not as well guarded, a surprise attack can win you the game on occasion.
Mavor Experimental Strategic Artillery: Although massively expensive, the Mavor can fire highly explosive shells with virtually limitless range. Despite the "strategic" in the name, these shells aren't actually nuclear, as far as we can tell, but they do offer pinpoint precision, in that you can fire at a target and be assured that the shell will land directly on it, which isn't something any other of the long-range artillery units can claim. The Mavor will also fire very rapidly, usually launching three or four shells before the first one manages to hit the ground.
Of course, the major drawback to the Mavor, and what will prevent it from ever getting built by anyone, ever, is the fact that it requires an insanely long time to build. A single T3 engineer will require six or seven hours to build it. With a mass requirement of almost 400,000 units, and an energy requirement of almost 12,000,000 units, you're going to need some serious resources to even try and build it. Even in the tutorial game, where we had three fully-upgraded ACUs and a dozen engineers working on it, it took 15-20 minutes to build at a constant mass drain of around 700 and an energy drain that went up to around 27,000 before it was finished.
The Mavor will win the game for you, assuming you can keep it protected, but in the time it takes to build, you could probably build a dozen T3 Duke heavy artilleries which would perform just as well, if not better. It's kind of academic, because this thing will never be built in any truly competitive multiplayer game
This list isn't meant to be all-inclusive; we're sure there are other small differences between these units and those of the other factions.
T2 Gunship: The T2 gunship of the UEF has a docking clamp that allows it to pick up a single small unit and transport it elsewhere on the map. Perfect for sneaking an engineer off to a secluded corner of the map.
T3 Strategic Bomber: The strategic bomber is equipped with light anti-air railguns, allowing it to shoot down pursuing fighters. It won't help much if air superiority fighters get on their tail, but it'll shoot down gunships and interceptors without too many problems.
T3 Heavy Shield Generator: The heavy shield generator of the UEF consumes more power than that of the Aeon, but has a much wider area of protection. It also winds up offering less protection than the Aeon heavy shield, however.
T3 Aircraft Carrier: The UEF navy cannot build the T3 aircraft carrier, which isn't a tragic loss or anything. If you really want an aircraft carrier, the experimental version isn't terrifically difficult to build.