Great question. This is the first time I have ever heard of Inkscape. Thanks for the tip!
Illustrator is the one used by professionals, but that does not mean it is better. Professionals have been using Illustrator for years, and the $600 or so for the software every few years is not a big expense -- especially when the company pays for it. Also professionals will have all their old files in Illustrator and will need compatibility with the Printing companies they hire and with other graphics software such as Photoshop.
If you do not have old Illustrator files to support, and if you do not have provide Illustrator files to a printing company, then Inkscape might be better. With Inkscape, you get the current version free. With Illustrator, you get an old version free.
I think the average person would be better off with Inkscape if it is as easy to use as Illustrator.
Here is a comparison of Inkscape vs Illustrator (based on the current version, not the free old version): http://www.brighthub.com/multimedia/publishing/articles/73024.aspx
Thanks for the info.