Whilst I agree to an extent about the whole 'genres are silly' there is obvious a fair bit of room between prog rock (at least in a strict original 70's prog rock) and metal, so there is room to discuss which you prefer (it's not we're arguing whether blackened-thrash is better than death'n'roll or something ridiculous like that). I consider Opeth a metal band, personally, who use some prog structures - mainly because a lot of prog bands rely on improv from jazz and/or more trained musical theory, whereas metal is usually set riffs and very little, if at all, improv). And besides, there are plenty of other influences there as well. But hey, that really isn't that important at the end of the day. What is however is the music moves you. I don't think I could choose one over the other though, it just depends what mood I'm in I guess.