They each have their strengths. If you're in Enterprise production, or scientific computing, you'll want RHEL or at least a distro based on that. Most enterprise, cluster and scientific software, as well as some hardware, is best-supported on RHEL-based distros, and the stability of these distros means few surprises during the disto's supported lifetime.

If you're hosting your own web apps on commodity servers, more into VMs and containers, or you are more concerned with tracking current versions of languages, kernels, and other software than you are with long-term stability, then Ubuntu can work well.