You need to consider if a triathlon bike is suitable for you on the specific Ironman course you’re racing.
The majority of pro triathletes find that time-trial bikes are difficult to handle due to the geometry, but they nearly always choose one because the positives far outweigh the negatives, by limiting their weaknesses to allow them to go faster over 180km compared to a road bike. There are advantages and disadvantages to using a tri bike, which vary depending on what type of rider you are.
When cycling uphill you’ll benefit from a lighter road bike, as the geometry allows you a more comfortable and efficient ride. The aerodynamic advantages of a tri bike are negligible when climbing and you may be forced to utilise the quadriceps more, which can hinder your run performance. A time-trial bike is more aerodynamic but you will be in a more uncomfortable position, which puts considerable pressure on your neck, shoulders, arms and groin, and although you may get used to this position it can still result in discomfort.
If you struggle on the hills and don’t have solid descending skills then I’d almost always recommend a road bike. The only exception is if you’re a particularly strong cyclist on the flats with otherwise fairly good bike handling skills.
A tri bike will generally be 1-2.5kg heavier than a road bike, so if your Ironman bike route is hilly then use your roadie, because the extra weight of a tri bike may take a lot more out of you for the marathon. Another thing to consider is gastro-intestinal issues: ask any of the best iron-distance triathletes and coaches what’s the hardest and most challenging bike workout that can lead to digestive problems, they will almost always tell you it’s a long ride in the TT position.
For gradients of less than 4%, the aerodynamic benefits can outweigh the disadvantages of the extra weight on a tri bike. See if you can go for a test ride on a tri bike before buying, and decide if it’s right for you. If you’re still undecided then I recommend you check out your Ironman course on both bikes. If this isn’t possible then ride over similar terrain, and observe how much you gain or lose on each bike. If you’re serious about the race and want to improve thereafter rather than just complete it, I’d recommend buying a tri bike.
Most triathletes benefit from using a tri bike, providing they’re properly fitted, complete key, long bike sessions on it and practise running off it straight after riding. Do your research and consider the area where you train, and make sure you choose the correct frame size and get a good bike fit.