It also depends greatly on what type of music you are playing and at what volume. I wouldn't get a magnetic pickup unless I was playing rockabilly or some other kind of louder more aggressive music. I've used a mic and a pickup combined and I make my choices based on the sound of the room and playing volume and band. Recently I did a three piece blues/swing gig using the Golden Trinity and my Realist pickup and ended up turning down the mic all the way just like your teacher said because the sound just wasn't happening and the pickup sounded much better. On the other hand I did a three piece wedding reception with piano and sax and I turned the pickup all the way down and just used the Golden Trinity because the sound was really good. There is no blanket answer. As bass players our sound is greatly affected by rooms and acoustics and we have to stay flexible, even on electric bass gigs.