On the other hand, the Debian ecosystem (of which Ubuntu is primary beneficiary, even though not the primary contributor) is probably about the same size.
There's one other important difference: many enterprise applications are certified for RHEL but not CentOS, therefore IT departments in many large organizations are not allowed by internal policy to use the latter for anything critical.
(FWIW, Ubuntu is always the same exact OS regardless of whether the customer hires Canonical for support or not.)
