Unit Types (UFO2000)
Unarmored (a.k.a. "t-shirt")
Unarmored soldiers have the 2nd worst armor of the game, followed only by Sectoids. And, just like the original game it doesn't provide much protection against plasma fire or explosive ammo. But you can still implement the old "send the cheap rookie first" strategy, or the cannon fodder tactic.
Standard Armor (a.k.a. "blue armor")
Blue armor can be quite useful and is medium priced. It can be difficult to kill the wearer with only one shot, which causes a better survival rate. However, when single handedly facing a power suit it will be at a severe disadvantage. Personal armor can survive long range fire from enemies but it is somewhat vulnerable to quickly suffering critical injuries, which will limit the accuracy of the soldier.
The big X-COM armor. Units equipped with this armor are walking tanks due to their ability to take damage. Power suits can't be easily killed in one shot if they have intact armor on all sides. It is common for a soldier in a power suit to take hits on several locations and still stand until the a hit strikes it at an already damaged location. It is nessasary to concentrate large amounts of firepower onto one location to kill them. The power suits are vulnerable at close distances because even their armor isn't enough to protect them from laser/plasma rifles' auto fire at point blank range. Their cost limits their numbers, which can be a disadvantage, in part because power suits can be taken down more easily if they becomes outnumbered by less-armored units.
Flying suits are power suits with the ability to fly and extra armor. The first characteristic can make them able to preform a variety of roles. However, cover is necessary to prevent enemy units to spot them on the air. Otherwise, if they're hovering on the open they are easy targets (the proverbial duck on the water).