From what I can see, the Ubuntu Wiki is more professional and more officially maintained, with less community articles by just anyone and more articles created by official groups.
It's also a hub for official information about Ubuntu and the related teams (LoCo teams info, Governance & Membership, etc.)
On the other hand, the CommunityHelpWiki is just that. It's a Community wiki intended for helping people with problems, as opposed to providing them with facts. It's also a bit larger, with tutorials being the main focus.
I think they are there to do different things - https://help.ubuntu.com/ has official and community documentation on how to use Ubuntu and do things with it - e.g. using the launcher. The community section is maintained by the community, and has various guides on different things (e.g Radoen graphics and drivers, Security etc.)