AFAIK CentOS does not support distro-upgrades (eg. upgrading from CentOS 6 to CentOS 7)[1], we always end up making a fresh install whenever we want to upgrade, i've tried to use the UpgradeTool a few times but it didn't work (there was always something blocking the upgrade).
I believe this would be a point for Debian, the upgrade between releases just work :)
However, this also comes with a perk. CentOS releases are generally the "current" release for at least three years, as opposed to two with Debian. Also, major releases are supported with critical security updates for years after they are no longer current. For example, CentOS 6 will still receive patches until November 2020.
Of course, whether this is important to you will vary based upon your needs. For some, the 2 year releases with an easy upgrade path between major releases will be the more important factor.
That being said, for RHEL 6 -> 7 I did a clean reinstall due to the large architectural changes and it was the right decision.
Yes, Debian is comprehensive, but it comes at a price. Red Hat/CentOS are much better at maintaining the (fewer) packages they support and providing timely patches.
Bugs in less critical Debian packages sometimes linger for years.
That being said, Debian is a great server OS and the only one except CentOS that I'd recommend to anyone.
1) No need for EPEL and similar extra repositories. Debian is extremely comprehensive.
2) Rolling updates. We keep some servers in the stable release, some in the testing release. No hiccups on upgrade, yet.
Servers you keep on the stable channel will go through the same kind of upgrades you see on major CentOS versions. I never had a CentOS upgrade go smoothly; We'd usually just reinstall everything from config management. When Debian 8 came out, we opted for a distro-upgrade and everything was perfectly smooth. It took longer to go through change management (cycle out of production, test, slowly reapply load) than it took to actually upgrade the servers.
The servers we keep on testing are non-critical, and go through much more frequent distro-upgrade cycles. We had a couple bumps on the road, during the sysv init to systemd transition, but overall I still classify the distro as extremely solid and much better than our previous experience with CentOS.