I have quiet big experiences with both environments.
We were developing distro based on CentOS 6.x between 2010-2014. The tools for creating packages, repositories, customisations are great! The base OS is super stable and I love that. If I have to do anything that needs to last a long time and needs conservative environment I would definitely go for RHEL based systems. Even some 3rd party developers prefer RHEL based systems - e.g. OpenVZ. We have been using a system based on Scientific Linux (RHEL clone) for vpsFree.cz since 2009 and it works like a charm. To add more, most HW vendors (like Dell) support only RHEL based systems. Even SW vendors for commercial OpenStack or other Software-define-* support mostly RHEL based systems.
On the other hand, Ubuntu gives you a lot of freedom in terms of combining semi-stable environment (I'm talking about LTS versions :-) ) and new features build-in (like ZFS). I love that you can combine multiple PPA and you do not have to build your own packages to have new versions. I know that there are SOME repositories for CentOS/SL, but they have their own problems.. At the moment, we have around ~1k servers running on LTS ubuntu versions with some custom based .deb, but luckily without any major OS modifications. The build ecosystem and .deb packages itself are far from the maturity you can get from RHEL environment.
To sum it up, anything with commercial support or super stable environment, I would go for RHEL.
For systems where you need new software and up-to-date versions, I would go for Ubuntu.