Illy will have some advantages if you don't already know Corel.
Corel has it's advantages too... & I don't mean to imply I could begin to list them all... but one is that it's cheaper then Illustrator... & the other big one is there are more
sign related solutions for plotting vinyl straight out of Corel.
The other varoius differences in the basic capabilities are minimal IMO... & the plotting out of Corel is not much value to a guy who uses Flexi.
That leaves price. If you can afford Illustrator, I think you should consider it first. As an industry standard program, you will always encounter Illustrator files in this field, both from designer files brought to you & from client requests for industry standard file formats of your designs supplied to them for their future use.
The main advantage besides price or plotting that I've heard over the years, is when people know the program already. If you don't, I think Illustrator is a better choice. (I don't really care that others will disagree... I just consider it a worthy suggestion for cajunsignguy)