FreeBSD is an operating system used to power modern servers, desktops, and embedded platforms. A large community has continually developed it for more than thirty years. Its advanced networking, security, and storage features have made FreeBSD the platform of choice for many of the busiest web sites and most pervasive embedded networking and storage devices.
Debian is very well suited for application servers, web servers, file servers, hypervisors (personally, we use Proxmox for some hypervisors and this is based on Debian), and much more. It can run on various CPU architectures and scales well from large to small. We have Debian DNS servers running on Raspberry Pis, and large application and database servers running on powerful Debian installs in our racks.
If you want an OS that is rock solid, easy to manage, and predictable, Debian might be the OS you're looking for. The official packages are generally far from being bleeding edge versions, but this means that you are using packages that are battle tested and will not break compatibility at any point during its life-cycle. This, to me, is one of Debian's core strengths and a good reason for recommending it to others.
On the flip-side, the lack of up-to-date versions of various packages may be an annoyance if you actually do need to be using the latest version of such a package. If you need to be using more modern versions, you will need to remedy that yourself or look at an alternative distribution. Ubuntu may be worth a look, in this case, being that it is basically Debian with newer packages and some minor differences here and there.
Personally, I would not recommend Debian for a graphical desktop OS. You'll nearly always be better off with one of the Ubuntu flavors for a modern desktop with all the bells and whistles you'd expect with a GUI desktop.
FreeBSD is the closest to a UNIX style operating system. This has been extensively used by vendors in the field of routing, switch and security as their base operating system. This speaks for itself being a reliable, scalable and secure operating system.
Debian is one of the most well-thought-out, logical, and intuitive Linux operating systems you can use. Configuration, package management, file system layouts, etc, are all based on many years of usage, and it's very rare to find anything surprising or confusing in everyday usage. Package management and updates are handled by APT in such a way that once configured, you can almost guarantee your system will behave as expected for the lifetime of its service. Top-notch system integration provides all the tools you'd expect from a modern Linux system, with all the stability you demand from a demanding server OS.
FreeBSD was the only operating system out of many I tried to install easily on older hardware and to run in a very performant way. For example, I had a lot of trouble trying to get Ubuntu to install on older hardware and when it did, it was too slow to use. FreeBSD installed quite easily and even after installing a desktop such as XFCE - it still run surprisingly fast. I was very impressed with it's performance, which it seems is a goal of the FreeBSD project.
