This is something we get asked a lot! The answer is not that simple...
If you buy new, you know that the bike will be in the best condition possible and is covered by a factory warranty. You can take advantage of the expertise from the shop staff to help you, to get you fitted and everything adjusted properly -maybe even get a service package as well to help you maintain your ride (if purchased in-store).
*Something else to consider is that we (Joe Mamma cycles) also offers great discounts on all the other goodies and gear you will need to get set up for the race season (helmet, gloves, pants and jersey to name a few) when you pick up your bike from us. So if you need more than just a bike, it may be worth it for you to choose new.*
When you buy used there is a great potential for a hugely discounted price, but you need to know the condition of the bike. If you are not comfortable with your knowledge of BMX bikes then you will need someone to inspect the bike for you (could be a friend that races or local shop). Even with an inspection problems can go unseen, and repairs can get expensive fast (a new rear wheel can range from $50 to $200 or higher!)
We recommend that unless you know the history of the bike and/or know how to inspect it that you may want to consider the long term cost of purchasing used upgrades repairs etc. If you do buy used have someone you trust check it over to make sure that you are getting what you're paying for and getting a great deal.