Full disclosure: I'm a Luddite when it comes to bikes. I've rode my 1993 Dawes Stratos road bike for decades and still love it but recently upgraded to a 1980 Velosport Prestige. (I love them both equally in case they're reading this.)
The question of which bike to own for the sport of triathlon is a common one. My short answer is whichever bike you enjoy riding the most. Life is too short not to enjoy our rides. Plus, when you enjoy riding you'll likely do more of it and get fitter and faster. Whatever bike you ride make sure it is well fitted to you.
Tri Bike or Road Bike
There is little question that with proper training most athletes will be faster on a tri bike; that is one built with a steeper seat tube (>76 degrees) and designed to be rode in the aero position like the photo above. The rider is more aerodynamic which means greater speed for the effort expended and that can translate to a ten to fifteen minute savings over 180k if the rider stays in aero. The more forward position also means the hamstrings are used less which can contribute to a smoother and faster transition to running.
There are a few disadvantages to using a tri bike though.
It starts with the price. You can't really go for an easy bike path ride on a tri bike so it means this is a second bike to own. The cost of tri bikes is generally a measure greater than road bikes so that means a considerable additional expense.
Speaking of the rides themselves, tri bikes are not safe when riding in a group since you're in aero position and not on the brakes. This means tri bike riding is more of a solo affair.
Next is the fact that riding in aero position limits your range of vision. Your eyes will be locked on the road ahead and not nearly as able to take in the beautiful world around you. And *PLEASE* keep your eyes on the road ahead. There is a very bad tendency for some in aero to put their head down to alleviate neck strain.
That 10-15 minutes saved in an Ironman is based on staying in aero. If it's windy or rainy, or the road is bumpy, you may spend more time in an upright position. Since the bike geometry is designed to be in aero, riding upright is not super comfortable. The tri bike is also not as comfortable for steep or long climbs as well.
The complex design features of some tri bikes (ie internal cables) can mean maintenance is more complex.
Clip-on Aerobars
A road bike with clip-on aerobars is a great option for some to consider as it allows you to ride comfortably upright but also gives you the opportunity to get aero for portions of the ride. In aero you won't be as comfortable or efficient as on the tri bike but the compromise does offer a solid choice to triathletes.
Food Storage
Some tri bikes have storage compartments built into them. If that's not enough space, or if you ride a road bike, you need to add storage capacity to your bike if you're planning to train and race in half or full Ironman events. Consuming sufficient carbohydrates during training and racing is essential for success. There won't be aid stations out there on your 180k training rides so you need to pack a fair bit of food. If you are consuming 60g of carb per hour that means you will need to carry with you as much as 500g of carb or the equivalent of 25 gels on a 180k training ride! Add to that the fluid you'll need. All of this means additional bottle cages and compartments like Bento Boxes. When selecting cages make sure they hold the bottle firmly so it doesn't allow the bottle to fly out when going over a bump. Cages that allow this are referred to as 'bottle launchers'. 🚀
Tire Width and Pressure
I'm going to leave this topic to the bike experts but wanted to flag it as it's a huge area of change in the sport over the last few years. On this topic and ones like wheel selection and crank length, do some research and talk to the team at your local bike shop. In fact, talk to your local bike shop team about ALL of the topics in this list. They are the bike experts and it's a relationship to nurture and value, just like your car mechanic.