Disclaimer-Any thoughts expressed in these pages are purely my own, unless otherwise stated.
There is no magic formula for choosing the right rod for an application. While it makes sense to want the lightest, stiffest rod you can get, that may not be the case. A spar that is too stiff can be brittle and break too easily. If it is too soft, performance can be lost, through frame distortion. It seems that the secret is to use a spar, or comination of spars, that is stiff enough to achieve the desired performance, while retaining enough flexibility to be durable.

Assuming that the same methods and materials are used, the only way to make a spar stiffer, while keeping the weight the same, is to increase the diameter. This means a thinner wall and less resiliency upon impact. Conversely, a smaller diameter spar, while being durable, may prove too flexible for the application.

Tapered spars can be used to concentrate stiiffer parts of the spar in certain areas. This can be varied even further by where the spars are cut. A full length, tapered lower leading edge, would be more flexible than one the had been cut. Tapered spars can also be used to alter the static balance of the kite, due to the thinner ends.

There many great spars available to us now. It can take a lot of testing to determine which
one best suits our needs.
I didn't include numbers in the "Relative Stiffness", as this chart is, well, made up. The numbers were arrived at by multiplying the weight by the deflection, to get a stiffness to weight ratio. In this case the lower the number, the less deflection for a given weight. The numbers are not used by any manufacturers, that I know of. They were done as a guide for me. I decided to include them in the site, in the hopes that they may help.