The reason most folk use fireworks is the ability to see your prototype working in any browser. Illustrator is a great tool for mock ups as you can use layers to build your prototype but unless you have the CS5 version with multiple artboards you can only export your pdf's in single pages and would need to use Acro Pro to develop it into multiple pages. Have a look at this tut it may help you make your mind up what tool to use: http://www.smashingmagazine.com/2011...e-illustrator/

Since this is a Xara related site why not have a look and maybe download a trial of Web Designer MX here: http://www.xara.com/uk/products/webdesigner/
You can use this programme to show your clients your work quickly and easily all you have to do is to publish your design as html without knowing any code host on some website and your clients can view it at there leisure.