I've been using Shatten Design "Dualie" soundboard transducers on 4 steelstrings and one mandolin, and tried one on my last nylon and liked it even though I never gigged with it. It's a snap to install too. This pickup is happier with a DI box and I tend to use a volume pedal and a Boss EQ. In a perfect world I prefer a shotgun condenser mic (it aims better than the large diaphram mic to avoid hearing me grunt and wheeze), but the pickup works in a pinch or as a sort of low threshold signal combined with a mic.
Experiment with placement, it can make a big difference.