4
I have used both XOOPS and Mambo to build websites and I can tell you now that given a choice I would choose Xoops. Simply put, Mambo is a bitch to figure out when you are new to CMS's. Mambo is designed professionally so you have to have some idea about what you indend to do, whereas XOOPS has the flexibility to be whatever you want it to be.
To test Mambo I created a test install and I played with various compnents and modules and it looked nice but it didn't feel right. Its got some nice features that I would love to seen in XOOPS but I like the customisation of Xoops. Themes are important to me, and Mambo's themes are pretty grey when it comes to looking snappy and eye-catching.
Yes, I've trid making my own themes to no avail. Its just too complicated at my current level of design.
If you still can't make up your mind, set aside and evening and install Mambo, add a few components and modules, have a poke in its guts and then decide what you will do.
People will probably disagree with what I have said here but, its down to personal preference really, and I found XOOPS my preference.