I'm the developer of OERPLib, I will try to answer your questions about it.
At first, OERPLib is not official, as the openerp-client-lib is (developed and supported by OpenERP SA, as it is claimed on the github page that you linked). AFAIK, openerp-client-lib was used in some controlers of the 'web' addon in 6.1 (apparently it's not the case anymore since OpenERP 7.0, but I could be wrong).
The goal of OERPLib is to provide a simple and well-documented API ("if it isn't documented, it doesn't exist", writting the documentation takes more time than writting the code, but it's a very important point for a library), and support as much as possible the different versions of OpenERP over the time. Currently, unit tests run well on OpenERP 8.0/trunk.
Regarding the features, OERPLib provides a simulated 'browse()' method and 'browse_record' (useful when debugging and inspecting data), a user-context automatically sent, keywork parameters to OSV methods, download reports, manage databases and so on... Also, an experimental support to JSON-RPC is implemented (just a low-level connector right now, but a higher level implementation can be achieved quiet easily, I'm just lacking of time to do it).
In the company where I work (ABF-Osiell), we use OERPLib a lot to process and import data, or as a debug tool when developing, or also to control/inspect on the fly complex data on customer servers.
And about developing, a new tool named Mandibule aims in some ways to be a GUI to OERPLib (it generates model relationships and module dependencies graphs at this time): http://mandibule.bitbucket.org/
So, to answer your first question: use the one you prefer :)