I have done some amaizing jobs using Codeigniter and forreal its very very good and cool and even lighter than SS, but SS is a CMS and Framework so no beef, i also enjoyed CI cause its both PHP4 and PHP5 based so make it flexible and hostable on most servers, and the Documentation is really the edge they have over all other frameworks
Light and Flexible, Silverstripe already gave us a CMS which is the sweetest part of it, though i have had hard times making the CMS totally customized but its still the CMS i have ever used.
For CI, they did an amaizing job, if we are to compare i think its not the CMS area we should only consider Saphire vs Codeigniter.
And there is no way you can download Saphire individually so that ends the topic, nothing to compare, CodeIgniter is a Framwork and Silverstripe is a CMS.
Or better compare Silverstripe to Expression Engine
But expression engine is only free for self use not commercial unlike silverstripe which is 110% free
Also i decided to test the Expression Engine CMS and i can simply conclude that SS is the best CMS and for CI for me is the best framework for all i have tried