In terms of musical theory metal and classical are completely different, metal theory is closer to early music in terms of style, mode etc. On a more subtle level we can observe similarities in the construction of metal songs and classical pieces, not so much in terms of structure (ABA ind of crap) but in the inter-relationships between ideas manifested as themes. I think classical theory leaves a lot to be desired in terms of its range of expression, it certainly falls short of early music and eastern music in this regard, so metal really has the best of both worlds.