Stems matter a lot in the stiffness equation. IMO*, more than bars. But you asked about bars.
For log event (TTs) IAB there are tricks with sticky tape or 3M 77 spray.
Flexy <> strong. think "unbreakable comb"
As in most things - It depends on application.
I like the ENVE SES although they are kinda pricey.
They have flared drops and you reduce the chance of hitting forearms on the bars. They are pretty stiff. The IAB position is comfortable. The cable routing is internal and exits rear near stem.
We bought them for junior and collegiate TTs which are IAB based as TT bikes are not allowed.
The Aerofly is very stiff and matt surface. Without tricks it will give better IAB adhesion. They also make a rise stem which offers a bar rise without stem or head tube stack rise. In extreme IAB you can put your forearms in the valley. MOst cannot do this. I'm just mentioning.
The Ritchey Solostreem is an integrated bar stem. There is a carbon plate under the bars to keep the cables in. The bars flex so much that that plate often (2 for 2) comes loose. It is a strong, but very flexy setup. Doing Paris-Roubaix good choice.
FSA makes stiff bars. They are not as flat on top and tend to be gloss. My kid does not use them, I do - not racing.
*I have had about 8 or so aero bars the last 4 years. I am not a racer, but my kid is and I am around a lot of those racer types. I am big (fat) strong - I just am, little to do with speed, and really notice bar flex. I re-purposed the solostreem for me on the tandem as they are areo and comfy.
Race with the aerofly or the SES. Choose stem according to application.
Last edited by Doge; 11-01-18 at 03:07 PM.