A properly fitting bike is more important than the bike itself. If your fit is off, and you can’t stay in the aero position for hours at a time without it _really_ getting painful in one way or another, it really doesn’t matter how fast or light the bike is…if you’re sitting up in the wind, when you should be down in the aero bars, you’re just going to be less efficient, and significantly slower.
If you’re buying a new bike, be sure that where you’re buying it is run by people who know and fit triathlete’s…like were I bought my Cervelo P2C from multi-Ironman, John Glover at Charleston Bicycle Company in Charleston, SC
…or to a great guy in your region like Jim O’Brien at The Right Gear, in Kannapolis NC
…or you can travel across the country to go to a famous guy…like Andy Pruitt at the Boulder Center for Sports Medicine, http://bch.org/sportsmedicine/default-sports-medicine.aspx in Boulder CO
Here are a few organizations that certify bike fitters, with further info on bike fitting:
And your fit on the bike can/will change over time, as your body changes…so always a good idea to have it checked at least once a season, and some would say, the beginning of the season and half way through. Do I always do that?…no…but it is a good idea. I had it checked a couple of times this last season, and it resulted in the seat being moved a bit more forward mid season…at Ironman Florida I was comfortably aero all the way…well ‘comfortably’ may be an overstatment, but no aches or pains, or anything that was bothering me, so that was good…very good.