Road bikes need to handle with agility in a wide variety of situations including climbing, cornering and descending. The seat tube angle is generally 73 degrees and the rider's position is often quite upright. Hands must have a variety of positions on drop handlebars to allow for easy shifting and braking. This position allows for maximum power transfer when pedalling, especially when you're climbing.
Tri bikes handle in a different way. They are made to go fast in straight lines while utilising rider energy efficiently and even conserving energy - you have to run when you come off the bike. In order to achieve this efficiency, tri-bike geometry has a steeper seat tube angle, usually 76-78 degrees. The top tube will be slightly shorter and the rear of the bike will be tucked under the rider more. This allows the rider's hips to remain forward and open in the aero position.
Holding an aero position on a tri-specific bike versus a road bike with clip-on aero bars should be significantly more comfortable, especially for longer rides. The forward position requires more energy from the hamstrings when pedalling. Hopefully, this will conserve some energy for the quadriceps when the bike leg is over and it's time to run. This seating position also improves aerodynamics - the head and shoulders are low and the frontal body area is minimised. Also, tri-specific bikes have features such as aero frames, deep-section rims or disc wheels, all of which increase the bike's 'slippery' factor, allowing it to cut through the air better.
If You Want Just A Road Bike
Pros A road bike can be used for all riding and racing - it's an all-rounder. The drop handlebar setup on road bikes offers maximum hand positions and good control. It's the way to go if you're just starting triathlon.
Cons Road-bike geometry is not as aggressive (or steep) as it is on a time-trial (TT) bike, which means the bike will have a more predictable riding style but not offer the efficient power delivery that can be gained from the position held on a tri bike. A road bike has a more upright seating position and therefore it is not as aerodynamic (or fast) as a TT bike.