I personally run Joomla. Joomla and Mambo split not too long ago. Mambo was operated and funded in part by Miro. Miro had different plans for Joomla than the developing team. When they no longer saw eye to eye the developers left Miro behind and started their own CMS branch which they named Joomla (with the help of the community).
Joomla has all the lead program developers and mostly all the add-on developers have sided with the original Mambo team and have migrated to Joomla. As said above the boards are more active and the code is going to develop quicker and more securely. Down the road the huge third-party support for Joomla means that when the two are no longer compatible, Joomla will end up with more add-ons (extensions, themes, plugins, etc). Some compatibility issues are already starting to show.
What it really comes down to Joomla has a much broader community of knowledgable people. And their help is quicker and friendlier. I’ve been on the Mambo boards recently and if anyone is running Joomla they refuse to help. You won’t find this type of behavior on the Joomla boards. Everyone is helpful.
And as mentioned above, getting a custom designed template isn’t all that expensive.
Or you can browse some sites that offer templates for free. Some pay sites have high quality templates as well.
This site has over 600 free templates for Mambo or Joomla
www.mambohut.com has another 300 or so.