I don't do a lot of illustration work, but when I do I use Freehand. For some reason, even tho' I use Photoshop and InDesign, I struggled through a project in Illustrator that just seemed to come naturally to me in Freehand. Feature-wise, they seem about equivalent although every upgrade seems to involve one app playing catch up and getting a little ahead of the other until the next upgrade cycle.
They both have free trials. I've always found the best way to learn is to just try it out.
Freehand
Illustrator