Conventional wisdom tells us that a stiffer material has less elasticity. That would mean that a stiffer frame, would feel not only more rigid, but also a bit more harsh due to less deflection of the frame material. That being said, depending on the layup, type of carbon fiber weave and resins used you could probably optimize the ride quality... that part I'll leave up to the Cervelo engineers to answer, because only they know what's under that shiny paint.
From what I've read, vertical compliance is a fine balance between frame design, type of material and how it is used. In addition, your wheel type, tire choice and air pressure will likely help you achieve the best possible ride for any given frame.
I'm sure the engineering team that worked on the S5 considered stiffness and vertical compliance in the design process and the manufacturing stage, especially since this was the bike that was meant to be the successor to the S3. Cervelo, likely produced a range of frames that feel fairly equal in terms of stiffness and compliance. However, the S5 seems to have been focused on its aerodynamics first, with the other qualities more of a price option passed on to customers with different budgets.
Back to stiffness and compliance though, frame design, which is the same for all three would have been the starting point, but more importantly the type of carbon and the layup/orientation of the material on the frame would be the final step to accomplishing their targets for weight, stiffness, compliance, and of course, price. If all the reviews are subjective and you can't test ride them all, you are going to have either go with your gut or get a direct answer from Cervelo Senior Engineer, Damon Rinard.
The three questions I would ask myself would be ...Do I want the lightest frame? ...Do I want the cheapest frame? or ...Do I want a black or white frame?