This
video comparison may be a good starting point. Also read this
freehand anonymous article (including the comments) on Creative Review. That should give you a good idea why some people prefer Freehand.
Having used Freehand for years, I generally find it to be more intuitive to use. Also FH has multiple page support (Illustrator's multiple artboards just don't compare), easier manipulation and selection of paths and bezigons, runs faster / less sluggish with less strain on the system, has a less clunky interface... etc, etc.
But it boils down to what you are used to, I'm sure seasoned Illustrator users would disagree. It's a bit like the Mac vs. PC debate.
I have both and use both. Illustrator is lot more compatible when it comes to final output for print. I think that is exactly why an Illustrator 8 EPS is the requirement for vector submissions on istock.