Illustrator vs Photoshop
The only reason I would use illustrator over photoshop is when I need to use vectors.
Say, I need some artwork that can be infinitely enlarged. That is when vectors are needed, because they are computed by the software on the fly and rendered, different from rasters which have their data fixed. That is why you can’t enlarge photos without getting them all blurry.
For my project I have a sequence that deals with flat design. It is definitely possible to work in photoshop alone, but I foresaw the need to up-res when doing a full HD film, so why not use illustrator and take advantage of its path workflow? Easy shapes, easy curves, easy scaling up!
If you haven’t seen my first experimentation with mixed media for my project, view it at https://aqsrelax.wordpress.com/2011/11/19/affinity-an-abstract-prelude/. I’m currently working on the revised version to be all ready for pitching (yes, we have to pitch our projects)!