[CentOS] mock, extras vs epel

I am looking to be a bit more standard in the builds of my CentOS rpms, and so I was about to install mock but noticed that there is one provided in the centos/5/extras and another in epel.

epel is obviously newer, but are there reasons/experiences in this group that would suggest sticking with the extras version instead? Or even reasons other than the shiny version number on the epel one to go with it?

Is the extras version the version used by the upstream provider and thus the CentOS team keeps it around to do the matching builds?

Thanks for the clarifications.