Are power chords 2 or 3 strings?
Two finger power chords sound very energetic and fast, so they are great for fast passages. Three finger power chords sound a lot fatter and powerful, so they are great when you want the song to sound a lot more brutal (\m/) or more primarily when you want the power chord to ring out.
How many strings are in a power chord?
For the most part, the two- and three-string power chords are interchangeable. Although for some situations, such as playing in the Chuck Berry style, the two-string version is preferable.