[eluser]Maglok[/eluser]
I have some Symfony experience and what seems to me to be the biggest difference is that Symfony is aiming more towards huge projects. It has deployment options, whereas (as far as I know) CodeIgniter does not. Not that I like those options, but they exsist.
It generates a lot, including the database schemes.
CI just works quicker and might also give off a vibe off 'quick and dirty' to new people. Whereas Symfony really forces a lot on you and should be a no go for small projects in my opinion.
CI is the Framework of my choice.