The concert ukulele, sometimes referred to as the alto, is just a little bit bigger than the soprano and some would consider it to have a fuller sound.
Because a concert uke is longer than a soprano, there will be more tension on the strings. This can be beneficial if you find yourself bending strings out of tune as you press your fingers down on the strings against the frets.
The frets are a bit more spaced on a concert ukulele than the soprano, so people with larger fingers might find it easier to play.
