Fedora
We use Debian and its derivative Ubuntu because the apt ecosystem and toolchain for Debian packages is far superior to the yum-based system used by Fedora and RHEL. This is large part due to a huge amount of investment into tools like debhelper/dh over the years by the Debian community. I haven't dealt with RPM in the last couple years, but every experience I've had with RPM is that the RPM tools are slower, have less useful options, and it's more work to package software for them (and one makes more compromises in doing so).
I think everyone has seen the better experience using Ubuntu in the shift of prevalence from RHEL to Ubuntu in what most new companies are deploying on their servers, and I expect that trend to continue as long as Red Hat is using the RPM system (and I don't really see them as having a path to migrate).
The experience with Ubuntu and Debian stable releases is pretty similar: A solid release every 2 years that's supported for a few years. (While Ubuntu in theory releases every 6 months, their non-LTS releases are effectively betas: They're often unstable, only have 9 months of support, etc. I wouldn't recommend them to anyone not actively participating in Ubuntu the development community). Ubuntu has better integration of non-free drivers, which may be important if you have hardware that requires them. But it's also the case that most bugs I experience when using Ubuntu are Ubuntu-specific issues, especially on servers (in part because Ubuntu has a bunch of "cloud management" stuff pre-installed that is definitely a regression if you're not using Canonical's cloud management products).
