Hi Fred,
We have a CI system which checks installs and upgrades on the following distros [all 64bit]:
- CentOS 6
- CentOS 7
- Ubuntu 14.04 [LTS]
In addition, I also test on Debian Jesse.
So, any of these should be fine and of course, seeing how CentOS is basically RHEL, RHEL 6 and 7 should be just as good.