Trevor is correct in his comparison to flexi. As an Illustrator user for 15 years, I would disagree about Corel being easier to learn. Because you have Photoshop, I think the similarities between both adobe products would give you an edge on learning illustrator. Corel is easier to afford, but Illustrator is an industry standard that is worth the extra money IMO. As far as your day to day needs either one will do just fine.
Flexi will do fine also, but Illustrator will come in handy for opening client files from designers & other Illustrator users (& opening corel files as well) plus you can legally load it on 2 personal computers you are using at seperate times... like a home computer for example. Some people take home a flexi dongle... but having Illustrator loaded at home is easier & does almost everything Flexi will do & more in many ways.
Also at the office I think you would find it handy to have it loaded on a design station seperate from the production computer Flexi is on. So if you have files being output from flexi... someone else could be preparing upcoming jobs elsewhere in illustrator, to later import into flexi for any final flexi-specific set-up work.
Also the seamless integration with photoshop is very convenient. My recent YP ad for example had 6 layers of information done in Illustrator. Then I could open it in Photoshop & still maintain my 6 layers which makes it real easy to add bevels to one layer, different layer styles to another layer, tweak the brightness & contrast on a third layers or whatever embellishments or improvements may be worth doing can be isolated to, or customized for the elements that need it.