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.
IBM AIX is a very powerful and extremely stable operating environment. It is well suited for applications that are business critical and cannot tolerate outages. It is best used to address large enterprise level application needs where stability and scalability are of paramount importance. IBM AIX is less useful for small enterprises.
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.
AIX is robust, helps the systems administrator, is built to prevent easily made mistakes. If you are used to other variants of Unix (in particularly Linux) there is no steep learning curve to get started with AIX. You need to learn the intricacies of the operating system, but that is true for any new operating system. AIX has built-in tools for almost anything you want and has the AIX Toolbox (on the web) for tools that are not included with AIX by default, but can be installed. Installation of AIX is modular, you can select with components and features you want to have installed. Installation of additional components (and usually also removal of installed components) is easy and straight-forward.
Compared to other Unix-like operating systems, IBM AIX is the one with a solid roadmap into the future and is likely to stay around for a long time (it is over 30 years old and still being updated with new features and components all the time). Compared to many Linux variants, the AIX operating system is more robust, easier to manage and well-supported by its vendor.
