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beware you are probably going to get 5 diffrent answers on what you should do. If there is one area of sound that is the most subjective it has got to be room acoustics and what materials etc.. achieve the best sound. Personally I think unless you want the room to be more soundproof than just using 5/8 drywall go for the soundboard, otherwise save your money. One thing for sure is that if you don't seal every crevise and crack tight you won't have a soundproof room. You can have solid concrete walls and if you don't have a good tight solid door it will be for nothing.

I would plan on treating the room itself with things like tube traps when you are finished, that way you can tune it in to your tastes.