power armor, as defined by starship troopers which founded the idea, is specifically designed armor that allows the user to survive in a multitude of environments. It offers greatly improved protection, since it is full body armor, and by increasing the users strength, it allows for thicker armor and heavier weapons. In starship troopers, the armor was used also because it had integrated weapons, electronics that range from medical to communications, and jetpack technology.
A power suit is a stripped down power-armor and is more designed to give the user limited capabilities at lesser cost, weight, and bulkiness. Essentially, a bullet-proof vest compared to modern combat armor.
Both of these definitions go from the original definitions as I understand them.