Pictures of professional triathletes mounted on bikes that resemble rocket ships grace the pages inside virtually every triathlon magazine. These images, while presented there to support the article or to advertise the product, serve an additional function, one that is not necessarily intended: they inspire budding triathletes to be just like the pros. As a result, when beginning triathletes enter the sport, they confront a multitude of decisions, with one of the most prominent being what type of bike to buy.
Generally speaking, as you first start out, never spend a huge amount of money on any gear, let alone a bike. Borrow one or buy one used. Ride a hybrid. Do something rather than spend a month's wages on a bike you may only ride once. Basically, try the sport first. If you fall in love with it, then invest in it.
Now, if you do decide the sport is for you, then you must choose between a road bike and a tri bike. The road bike is normally the best choice, especially for the recreational triathlete who competes for himself, as it is built to be considerably more flexible than a time trial bike. Road bikes work better when riding in packs, cornering, and climbing, and they are constructed for a more upright position, making shifting and pedal transfer easier. Therefore, the bike offers quite a wide range of services for any rider. Conversely, a tri bike, made for flatter surfaces and predicated on the need for speed, forces the rider down into the aero position the majority of the time. The positioning is less comfortable but far more aerodynamic, thus creating faster times. The geometry of the tri bike also places more emphasis on the hamstrings during pedaling, which leaves more energy in the quadriceps, which is of critical importance when transitioning to the run.
Overall, you need to evaluate your goals and commitments. If you are dedicated to triathlon and want to compete and train regularly, then a tri bike may work for you. If you want to ride leisurely with friends and compete in some smaller, less rigorous events and train part time, throw some aero bars on the road bike and stick with that.