Usually when buying used, if you're not an expert, you'll have to have a bicycle person with you, who can evaluate the actual condition of the bike. The bicycle could have been in an accident and have frame damage. It might have mechanical issues. It may be the wrong frame size for your body dimensions.
OTOH, with a new bicycle, your get a warranty and generally free mechanical adjustments or repairs for some specified time period. The bike shop salesperson will make certain that you have purchased the bicycle that best suits your needs and body dimensions. You will also be properly fitted in most cases. You can rest assured that most likely your bicycle has no frame damage or mechanical issues.
* Don't buy a bicycle simply because it fits your budget. Buy a particular bicycle because it suits your riding habits, your riding terrain, and your budget, as well.
Road bikes are for riding on mostly smooth paved roads and possibly racing upon those roads. Mountain bikes are for uneven dirt trails, gravel, rocks, and roots. Hybrids are made for any type of terrain between paved roads and those mountain bike trails. Ideally, it's best to save up the most you can upfront and then make your purchase, afterwards.
Used bikes generally require more expense to place them in tip top working condition. So just keep that in mind, before you make your decision.
Last edited by Cfiber; 05-06-13 at 02:02 PM.