I don't know much about Mambo, having never used it myself. However, I would personally recommend against using Joomla!, as I've had many problems with it in the past (mostly, it seems to be rather buggy and caused a lot of issues with permissions - often setting permissions on files so that they could be edited by "nobody" - including me and even Joomla! itself), and didn't find it all that user friendly. In addition, it produces code heavily laden with tables.
I would recommend that you pop over to OpenSourceCMS - a site that lets you test out various popular content management systems (including Mambo, Joomla!, PHP-Nuke, Drupal, Textpattern, and many others) without having to install them on your own server, so you can get a feel for a few different CMSs and decide which one(s) you like (and, of course, you think you're client will like).
It may even be worth your while to get together with the sales rep and your client so that they can try out a few different systems themselves (though you may want to narrow it down to two or three you feel comfortable working with first) - that way you'll know they'll be happy with it.
Also: keep in mind that while not all CMSs will necessarily be able to meet all of your needs out of the box, a little tweaking can do wonders.
Hope that helps!
-Patrick J. B. Simmons | @hobbramble
"If I could give you one piece of advice to remember for the rest of your life, it would be this: when Patrick tells you to do something, do the opposite." - Nolan Ladewski