Triathlon - Smaller Tri bike?

I am looking to buy my first bike, with the intention of doing tri's. I was going to get a road bike at first, but I have found a great deal on a tri bike. (Cervelo Dual with Dura Ace)

My question is do most people ride a size smaller tri bike compared to their road bike?

(I am 6'4'', and have been test riding road bikes that are 60cm. The tri bike I am looking at only comes in 58cm, and there is no way I can test ride it. I would plan on getting it built and then professionally fitted if I bought it)

Yes, most people ride a size or two smaller than their road bike. But with the different way bikes are sized, I would highly suggest riding the bike or something similar before buying it. I have seen too many people lose big money buying bikes blindly.

See if the shop where you are going to get your fit has a Serotta fit bike or something similar. Have them set it up like the bike you are wanting and see if it might work for you.

Yes, most people ride a size or two smaller than their road bike.

another newbie question: Why do people choose a smaller bike? Is there an advantage despite possibly a suboptimal fit?

To get an optimal fit most need a smaller size Tri bike. Head over to Slowtwitch and read all about FIST fitting to learn the differences in fit, but the short answer is you need a smaller top tube becuase you are sitting much more forward and you need a shorter head tube becuase you are a lot more leaned over and there needs to be room for armrests.

60cm CAAD8, 58cm Dual

I'm 6'2" and have a 34" bike inseam

You kind of have the process backwards, you need to get fit, then buy your bike according to the fit, not buy a bike and then get it to fit. Even if its a smokin deal, if you are compromised getting it to fit, then it won't be a good deal.

I was originally going to go with the cervelo dual, but I have found a 07 kuota k factor bike for around 1600. this would be a complete carbon frame with dura ace. I have read that kuota has a more relaxed tri geometry, that I feel would fit my bigger body better than some of these aggresive postions of other bikes.

any feedback on the kuota k factor?

Didn't you ask the same question in another thread?