What are Zertz inserts on a Specialized Vita?

[Seajay]

The Specialized was designed to be comfortable over long distances and had 25mm tyres as opposed to the 23mm on the others.

Even better if you could use the same wheelset with both bikes (since the wheels and tires make a difference

Well noted, Sardine, and PLL
The change to the 25mm from a 23mm tire (and appropriate pressure) on any bike will override any other comfort "feature". Not sure why folks are feeling the Zertz bikes as less responsive. No reason they should feel that way. Personal preference I suppose.