(answer by FFH Mark)
For me, the simplicity of Freehand is the difference and that equates to Creativity and Speed. Illustrator is like a swiss army knife; it has the blades and can opener also corkscrew, toothpick, scissors, etc. These are tools needed sometimes but can get in the way of the knife blade itself or it keeps the knife cumbersome and heavier than I want.
I prefer Freehand because I can move very quickly and in multiple pages of various sizes. I can shuffle objects from page to page and into the margins to create a poster, postcard and brochure all in one file. Especially good when designing variations on a theme that are mixed sizes.
"Paste Into" feature is brilliant and simple unlike Illustrator's clipping path. I can edit my collected "Paste Into" object no matter how many items I've pasted in it. Again, it's a way to stay creative through simplicity.
The number one difference is the selection pointers which are simpler and require less steps, but if you are used to Illustrator; this won't make a difference to you. There are other tools in Freehand but I suggest you download and play with the trial version at Adobe: http://www.adobe.com/products/freehand/
Also Alberto, I keep my Illustrator CS3 close by. Sometimes I need that "Swiss Army Knife" for the Freehand files I import into it; to add special features. I need a corkscrew or a nail file every so often too.