What's the difference in sound, between an acoustic guitar with a soundhole magnetic pickup and an electric guitar? I Know that el. guitars usually have more than one, but is that all? Does the hollow body contribute somehow to the sound?
A magnetic sound hole pickup for acoustic guitar reproduces sound waves in exactly the same way that a magnetic pick up on an electric guitar does. It uses magnets to pick up the vibration of the strings and translates that through an electrical signal sent to the amplifier which sends those vibrations to the speaker in the amp or PA speaker. There are more controls on board an electric guitar to shape and color the sound and usually different pickups with differences in the way the magnet is configured - that will alter the sound differently than the other pick up or pick ups.
A magnetic sound hole pickup on an acoustic guitar will actually pick up more volume from your wound strings if you use electric guitar strings which are wound in nickel (which is magnetic) verses acoustic strings which are wound in bronze (which is "non magnetic"). With acoustic strings, a magnetic sound hole pickup is only reacting to the vibration of the steel core of an acoustic string. Try it if you need convincing.
Having said that, there are several factors that make an acoustic with a magnetic sound hole pickup sound much different than an electric. For one thing, an acoustic guitar will provide some sound from the guitar itself - acoustically. Folks a good ways away may not hear nearly as much of that as the folks sitting close to the stage.
But beyond that, most sound hole pickups, also have sensors built in that detect some of the vibration of the guitar's top (also known as the soundboard) which is what produces most of the acoustic sound of an acoustic guitar. The sound-hole pickup is clamped to the top and feels the vibrations. Some sound-hole pickups such as the LR Baggs M-80 - get almost as much of their sound from the vibration of the top as they do from the magnetic field of the strings.
Also, sound hole pickups are designed for acoustic guitars, and therefore intentionally configured to attempt to replicate or mimic the sound of an acoustic guitar. But unlike other systems designed to allow you to "plug in" your acoustic guitar (such as body sensor pickups or under saddle piezoelectric or in the body microphone systems) it does not actually reproduce the acoustic sound.
Obviously, there are several non magnetic systems designed to amplify an acoustic guitar that do not use magnets to pick up on the vibration of the strings themselves (see examples in preceding paragraph). Instead they translate the vibration of the soundboard into a signal that is sent to the amplifier.
Which system sounds best on a particular guitar with a particular amplifier or PA, is largely a matter of personal preference. Your choice will also depend on if the acoustic will be played solo - or as part of the overall mix with other instruments.
Ummm. Like Carl noted, it can not really be answered.
Acoustic guitars are acoustic. They can be played loud enough without an amplifier. All the pick up is doing is amplifying the sound of the strings resonating, on an electric guitar (solid body) if you have 3 pick ups, each pick up contributes to the 'tone' of the guitar. Typically on an electric guitar there is a switch that switches between the pickups. Top is closest to the neck, middle typically picks up all 3 pick ups and the bottom position typically get the bottom pickup.
The closer to the neck of the guitar on a solid body electric is usually a 'deeper' more bass tone, middle is a more mid rounded tone and the bottom pick up is typically more twangy or higher in tone. Inheritly because of the position from the neck. Then from the amplifier/speaker you would adjust tone from there.
A sound hole pick up for an acoustic will just get one signal from the guitar, Because of the one pickup, you would have to adjust the tone from the amp itself.