A set neck is glued into the body of the guitar. Although it's a matter of opinion, I think most people would go with the set neck as being stronger, but both can last a long time(you see a lot of old strats still around). The biggest difference between set neck and bolt on's, is sustain. Set neck guitars generally will sustain much longer than bolt on's(this is a big part of the Les Paul tone). This is because the glue fuses the body and neck together. This way, the neck is resonating, as well as the body, thus more sustain. Hope this helps
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