as an (most times) just end user
of EL6/7 I'd like to say that I would
prefer bugzilla.redhat.com for centos bugs
It makes shifting bugs between RHEL and CentOS
very much simpler.
The reason many bugs get reported first vs
CentOS ist people do not really know the relationship
between RHEL and CentOS.
And the people who know the relationship
have often no license to test if it's a CentOS
or RHEL Bug (you would need a RHEL license
to try to reproduce).
When you have some experience you can of course
tell with a high probability if it's CentOS
or upstream related.
TBH: Most (upstream)bugs tend to get closed anyway
if someone finds out your not using RHEL.
(This get's masked via sentences like "Please
contact your support channel if you want to raise
the priority of this problem")
I wish RedHat would improve this behaviour, but
you can not really argue if you get it for free.
Back to the Bugzilla Question:
I really think bugzilla has way better features and
usability than mantis, but that's just me, and of course
you have the ability to shift a bug to epel/fedora/rhel if
it turns out you reported it against the wrong product.
I think this might be increasingly important when
you look at all the sig products which incorporate many
projects already using bugzilla.redhat.com like ovirt, glusterfs
and so on.