I think it depends on what type of music you like to play. For Classical pieces, you need nylon so that your fingers won't get hurt too much with plucking. With pop or rock music, you can have the steel string.
I would probably go for an extra guitar so as to avoid having to change the strings from time to time. In this way, and irrespective of the kind of strings you go for, you will be able to play any kind of music you want to.