T3 Anti-Air SAM Launchers or just SAMs are the highest tier static anti-air tower and also the most expensive. These should be built to supplement T2 Flak AA rather then replace it.
SAMS are good defenses for your main base as well as other things you would expect T3 bombers to attack, as no non-experimental unit can survive being hit by multiple SAM rockets (gunships are an exception). There are inconvenient things though such as the fact that SAMS have a slower reload time and target a single unit, thus SAMS are prone to overkilling a target and aren't as good at taking out swarms of low-tech units as flak is.
SAM positions are easiest to destroy from the ground since they don't fire at ground units, however they are almost always defended. If you encounter a SAM that you can capture easily though it is almost always good to capture since where it's placed to be an inconvenience for you it will now hurt your foe. The other way to destroy a SAM is by swarming by presenting too many targets for its single target rockets to shoot at, or, and this is extremely costly you can send a first wave of cheap units to distract it and then hoping the second wave comes fast enough then destroy it with better units in wave 2.
In the original game, SAM launchers have the same range, vision, buildtime and cost but differed in health, DPS, and projectiles. The Cybran Myrmidon has less health but does more damage while the Aeon Transcender has more health but does less damage while the UEF Flayer sat in the middle.
Each SAM launcher in Supreme Commander: Forged Alliance has the same health, range, DPS, vision, buildtime and cost. The only difference is in the projectile – specifically, the number and speed of them. Note that all missiles are single-target, unlike Flak units.
The Aeon Transcender has a low rate of fire, but highest accuracy - making it the most effective at taking down both types of advanced aircraft: fast ASFs, and "large" aircraft such as experimentals and heavy gunships. However this means it needs the most support when pitted against weaker aircraft in larger numbers.
The Cybran Myrmidon has a higher rate of fire. Its missiles are fast and accurate, although not quite as fast as the Aeon's version, the number of them more than make up for it.
The UEF Flayer fires the most projectiles. Its missiles are not as fast, nor as accurate as either the Aeon, nor the Cybran versions, it is still a very respectable structure in its own right. It will work acceptably against large numbers of enemy aircraft and is perhaps the least likely to need backup flak for area of effect, but will still operate best with them for scaling damage support against large numbers of slower aircraft (such as a low tier spam).
The Seraphim T3 Anti-Air Defense is widely accepted to be the worst of the four. While it has the same DPS, its projectiles do not track, leading to a far, far lower Effective DPS. However, while it may be inferior against fast fliers such as Spy planes or ASFs, it's high muzzle velocity increases the chance of killing Strategic Bombers before they have a chance to drop their bombs. It's stats are the same as it's seraphim counterpart, but it has tracking projectiles and does not immediately show up on radar like seraphim structures, making it superior in every way.