Well, steel strings have been proven to cause callouses. But, then, so have plastic strings so I suppose that doesn't differentiate. I prefer a steel string guitar because I can make more noise and misplay a wider range of music on it, from Buddy Holly to Leadbelly to, if pressed, Ray Davis (I'm not sure that that is very far). But, I must admit, when I listen to good classical guitarists that I am amazed at the nuances that they can tease out.