I use both, and both are excellent tires and both have their merits.

Depends on the wheels. Hed 3 go with 19 Bontrager X Lite. 2008 and new Zipps, use 21 Zipp Tangente.

thanks for the advice. I have been debating over this exact thing!

It has to do with the rim shape. Talked to Zipp on Friday for about a half hour on this. They were REALLY helpful.

Because of the bulge on Zipp 2008's and newer, a 21 is the best way to go as it sits flush with the rim. The Hed 3's have never changed shape and don't have the bulge, so a 19 works best aerodynamically speaking. I picked up the Bontrager X Lites (R4's were not available) for my Hed 3's and I always run Zipp Tangente's on my Zipps. I have been getting pressure from one of my mechanics to give schwabble another try (really bad experience in 2008 with blowouts).

so if you have fat rims.... run the Tangente's... narrow run the Bontragers - is that a safe theory?

Pretty much what Zipp said. And they didn't discourage me from other wheels, but did add in that the new 808 Firecrest is faster than their 1080 clinchers. And that the 1080 Firecrest won't see production until late 2011 or 2012.

this is my setup:

zipp 700x21 on a fp60 front, zipp 700x23 on a powertap velocity wheel with cover.

the 700x21 i have never raced on, the 23 i have. great tires. the 21 sits flush with the wheel.