Unit Types (UFO2000)
Standard Armor (a.k.a. "t-shirt")
T-shirts have the 2nd worst armor of the game, with only sectoids faring worse. And, like the original team it doesn't help much against plasma fire or explosive ammo. But you can still always stick to the old "send the cheap rookie first" strategy, or the cannon fodder tactic.
Personal Armor (a.k.a. "blue armor")
Blue armor is quite useful and medium priced. It can be very hard to kill it with only 1 shot, which allows for a better survival rate. However, when single facing a power suit it will be severely disadvantaged. Personal armor can survive long range enemy fire but it will also suffer critical injuries and limit the accuracy of the soldier.
The big X-COM armor. Units equipped with this armor are walking tanks because of their ability to take damage. Power suits with can't be killed in one shot against intact armor on all sides. It is common for a power to take hits on several locations and still stand until the second hit finally kills it by striking at an already damaged area. Being able to concentrate large amounts of firepower in one location is essential to kill them. The armored suits also work better at long range fights because even their armor isn't enough to protect from laser/plasma rifle auto fire at point blank range. And their cost limits their numbers, which can be a disadvantage. A power suit can be taken down more easily if he's outnumbered by less-armored units.
Fly suits are power suits with the ability to fly and extra armor. The first characteristic can make them great for a variety of roles. However, cover will be necessary to prevent enemy units to spot them on the air. Otherwise, if they're hovering on the open they make great targets (the proverbial duck on the water).