There's not an easy answer. The highest gain before feedback (GBF) is from a piezo transducer; those also have the worst sound. Mics are vastly better but with more limited GBF, especially in challenging situations. There are several systems that combine a mic & pickup, those are arguably the best compromise, but many of them supply a mic inside the soundhole, which I think is among the most terrible places possible. Separate pickup/mic solutions may be preferable but more cumbersome.
Freestanding mic vs. mounted on instrument: soundboard-mounted can work quite well and allows more freedom of movement. The tone is usually the best. However, the entire soundboard becomes a boundary surface (which is a big factor in the natural tone), so there really is no off-axis rejection for anything in front of an instrument. If you have to have a wedge with your guitar in it, they probably won't work; you'd need a directional mic with its null aimed directly at the wedge (which also has its limits for GBF, as you'll still get reflections off the stage floor and instrument into the mic). Soundboard mounts work very well where the stage is all acoustic and reinforcement is only needed in the mains.
