Linux OS Based distribution is very subjective matter according to personal taste. In most case we are uncertain to choose “right” distribution because there’s pro and cons over the internet. I will cover the reason why choose Debian for a server while it sounded subjective. I am not talking about desktop environment, Ubuntu wins down.
Ubuntu Server is one of several sub distribution from Ubuntu family. It’s easy to use and many people use it as OS for the Server. Debian is Ubuntu ancestor, literally. Debian could be as server OS too and have no distinctive category or purpose, Debian doesn’t have special built distribution for different purposes excluding Live CD. It’s only have one form, Debian Installer. That’s it.