Well I use Steel-String. I've never really tried the nylon-string before. I heard its easier for plucking.
Steel strings are for steel-string guitars, and nylon strings are for classical guitars. It's theoretically possible to use nylon strings on a steel-string guitar (although I've never heard of anyone actually doing this), but it would be a very bad idea to use steel strings on a classical guitar because they could easily warp the neck.
metal strings are good for pop, country or rock music while nylon are for classicals or spanish music
different strings respond better to different wood types and guitar design
some artists prefer metal because it requires less attention and maintainance and its cheaper too while I prefer nylon(though I will have to change it every week or 2) mainly because of its deeper tone
and it doesnt damage my right hand nails