Just out of curiosity, why migrate to CodeIgniter? If it's an older Symfony app running on the 1.x branch or something, it would seem easier to migrate to Symfony. Since you're new to CI also, seems like the more logical step might be keep it in Symfony and go from there?
I mean, either framework is fine, but there are huge differences in architecture that will probably make the switch a bit of a pain, and, if you have to learn a new framework anyway, learning the latest version of Symfony seems like the easier of the two paths.