go for joomla many are using it so you'll have more chances of support,and yea joomla is better then mambo though they are almost same.
joomla is just the next generation of mambo. using mambo would be like using windows 95 instead of vista. it also has many security holes.
mambo and joomla core files are same but this days joomla developement is more.Mambo hase admin login probe for many members we need root access to fix it.Joomla forum is more active with lot of themes and extensions.So go for joomla.
joomla by far... much more support and more secure, not to mention most all of the original guys who made Mambo what it was.
I love mambo and I am now using it, it is so cool for a niche site. take a look if you still have hard time to chose www.top10s.net
Mambo is an older version of Joomla in my opinion. I'd go with Joomla. I've heard "ModX" is also a good CMS. In regards to praectorian guardian's comment, Joomla is actually very good with SEO. They have plugins to really optimize your site. As the matter in fact, Joomla is just as good if not better than anything else for SEO.
I'd use Joomla. I have had an EXCELLENT SEO experience with Joomla - search engines are attracted to it. My WordPress sites maybe get visited once a month, but my Joomla sites are once a week or more. - Edward 602
Hmm maybe with extensions/plugins Joomla would be good for SEO. I recently (after reading people on here) used joomla for a test site. Really it took a very long time to setup the site how I wanted it to be. I hate the admin system.. too clunky for me, it reminds me of AOL kind of. Perhaps I should have installed some plugins or something.. Is there a something that can change the auto installed version to one that doesn't have all kinds of crap I have to turn off? Last time it had so many things turned on to make the default page look good.. just made me annoyed since I didn't want polls, or online users to be displayed. Then again I did eventually get everything sorted, and installed looking and feeling the way I wanted it. Now I have my site up and running, I imagine it will be much easier next time.