There are many decent membership site scripts on the market. It depends on the level of membership you desire, whether you want different prices for different types of members, and so forth. I run a nice membership site script on my membership site.
I prefer Joomla as it has lots of useful Mods. And easy to customize, try looking at core files, those ARE 80% opensource.
I am not sure if the other people get what you mean. What are you planning on using if for? The two best scripts in my opinion, are the Launch Forumla Marketing script (LFM) by Robert Puddy - or the amember script. The LFM has a lot more features and is really equipped ready to go from a marketers perspective, with inbuilt OTO deals and IPN integration. Not sure if it's public knowledge, but I think Robert Puddy is having a sale in September for the LFM script too - u might get a nice deal
I would go with Joomla. The only downside to joomla + community builder is that you have to update community builder when ever joomla gets updated and then all the plugins with community builder might not work right and the list goes on and on. Remember to: KISS
Make sure you understand how you are going to monetize your site if you are going to use Community Builder... The current CB users registration is not the same as Joomla user registration so the paid subscription plugins are limited compared to just using joomla. I'm using both joomla, amember and community builder here: http://www.MySmallBizU.com but I'm protecting video links, not the content managed by joomla in the site.