For those new to these extensions, Community Builder is one of the oldest (and possibly most used) Joomla extensions. It has a plugin system and lets you quickly develop user profiles for logged in members; with custom fields and member lists. Because its Open Source and how long its been around, there are a ton of plugins to give you google maps of your members, on-going/live (facebook wall-type) listings of member activity on their profiles and more.
Jom Social isn't Open Source. Its a commercial release from the Malaysian web development firm Azrul Studio. They're well known for simple blogging and commenting extensions and have recently released Jom Social - which pretty much gives you the same functionality as Community Builder for $100+ but doesn't require must setup/configuration once installed.
There's been discussion in the Joomla forums about this topic before and I think people are most concerned about the lock-in factor of using commercial software plus the question of how integrated JomSocial is to Joomla itself.
