The warranty issue is the only downside to buying used (that and the fact that you never truly KNOW what the previous owner did with the bow). That being said, bows depreciate faster than vehicles, so I prefer to let someone else take the hit, and buy used.
With it being your first bow, however, I would try and find a reputable shop near you. They will be able to do a lot for you from setting you up properly, answering any questions you may have along the way, provide accessories, build you arrows, etc. Plus, you're supporting a local shop, and getting a warranty (if purchasing new).
It really boils down to what you prioritize. Would you rather save some money, and go at it by yourself, or maybe spend a little more and retain a valid warranty?