Which one do I buy as a first time cyclist trying to train for an Ironman?

I would get a triathlon bike honestly.

If you are this fresh and don’t know much about bikes, I’d probably buy from a shop. There are hundreds of used bikes with varying components and maintenance required. 

If you aren’t familiar much with any of that, I wouldn’t jump right in while also trying to train for an IM. You may even want a dedicated tri bike which opens up an entirely new group of bikes to pick from. 

There’s just way too much to choose form if you don’t know anything about what you want, saying nothing about what fits you/your style best