Let’s start by dispelling this myth; people who play BASS with a pick rather than their fingers are still BASSISTS, not guitarists! There is no wrong or right, if it gets the job done then that’s all that counts.
That being said, there are some major differences between both techniques…
The most notable difference is the tone produced. With a pick you will get a lot more attack to each note due to the hard, stiff nature of a plectrum and subsequently there will also be less bass frequencies coming through.
You can produce a more ‘bassy’ tone by using a technique known as Palm Muting (which is covered in more detail in the ‘Open String Muting’ lesson (coming soon)). This is achieved by placing the palm of your picking hand across the strings, close to the bridge, dampening the note and killing off some of the higher frequencies. This is due to the fleshy nature of your skin, as opposed to the hard plectrum.
Playing with fingers will give you vast amounts of tonal control over your playing compared to playing with a pick. You can achieve a very ‘pick-like’ sound by plucking the string very close to the bridge or get a very vintage, dubby, warm sound by playing nearer (or in some cases over) the neck.
This is down to the way the string vibrates at different points along its length. The nearer to the bridge you pluck, the less the string will vibrate causing the bass frequencies to be dampened (as they require more energy to be audible) and the treble frequencies are perceived as louder.
Another difference is the way in which they allow you to play. Generally playing with a pick will allow you to play faster than fingerstyle although some extreme metal bassist employ the use of their ring finger on their plucking hand to give themselves an extra speed boost. I have also noticed a difference in the way I, personally, am able to perform live on stage between both methods. I find that playing with a pick allows me to ‘rock out’ more than playing with fingers but at the sacrifice of the accuracy and tone I achieve from playing fingerstyle.