Not very helpful, maybe, but I use both... but not at the same time!!

For me it depends a lot on the song. I mostly use steel (and 12), and tend only to use nylon when I want a specifically 'Spanish' sound.

Gerry Kearns of the Oldham Tinkers has always used nylon, though most other performers (as mentioned above) tend to prefer steel. If I could only have one guitar with me, it would be steel.

One problem with nylon strings is the time taken to 'bed in' (stay in tune) after they've been changed. It could be a big problem if a string broke during a performance...