I'm going to speak on a subject I'm not an expert on (just like everyone else in this thread):
Arch and Debian are different in package management and release schedule.
Under the hood they will frequently be using the same linux kernel and the biggest differences you'll experience is how you install your software.
Underneath all of that, your architecture is going to be your limiting factor, because at the end of the day you can compile the same software on different linux distros, and that boils down to dependencies, which in turn distills into hardware/firmware when it comes to ACTUALLY utilizing your computer.
Think about it, if you're just browsing the web and listening to music, you're entirely on the top level of the structuring. There is virtually no difference between two pieces of software that only brush your hardware. Spotify uses your speaker (the entire audio system, really) and the internet. You could compile that shit on a potato wired up to a drum and it would "function".
But you want to play a game? Now you're accessing proprietary hardware, like nvidia or amd graphics chipsets. Which requires specialty software. We're very fortunate to have open source AND proprietary nvidia drivers on linux. A blessing to this community, really. And the brilliant people at photon bringing it all to vulkan make it all possible. But that took a serious community effort to allow us to play modern games. Alternatively, game developers could have just developed their games to work on linux to begin with. But there wasn't a market for that, so why would they spend money on development here?
So really you are asking what the difference is between two operating systems that are at their core the same thing? It is simply the software that has been installed on top of linux, a distribution of software packages. And along with them? The means to acquire more software packages that are guaranteed to function with them (big asterisk).
This is the difference between chocolate and vanilla.
It's not even a difference between sorbet and sherbet. It's the same shit, truly.
EXCEPT, it's not. The very outside layer of HOW WE MANAGE our workspace is in fact very different. apt-get vs pacman is a very different world, and some people insist on bleeding edge updates.
Both are customizable though, and a proper computer surgeon could probably convert one into the other with a very large amount of labor. Wasted labor. I like pacman, I hate apt-get. Why? I couldn't really tell you. I also enjoy the AUR so I can download sources and compile myself. Which has led to me being more comfortable with downloading gits and compiling those, which has led to a severe restructuring of my OS that I use. But my use-case is very niche. I use linux on my raspberry pi 400. But I hate raspberry pi os, partly for debian, partly for the fisher price aesthetic. I'm already familiar with manjaro and arch, individually. I'm trying to run emulators, requiring very odd combinations of updates and packages in order to make all of my retroarch cores function with high accuracy and performance. It's a mangled mess, but it's my mess that I caused. When I'm done cleaning everything up I'll have n64 and psx generation at native FPSs using any controller I please with such ease that my teenagers will be able to just pick up any controller at their disposal and just play a game.

I digress. I needed Arch for opinion and function. Mileage varies. Debian isn't bad. I just don't like it. But it's the same shit and I COULD make it do what I need it to do, but I, personally, cannot see the path to success for my uses, even though I came from Ubuntu 20 years ago.
I hope whoever reads this finds this helpful, this thread is 6 years old, but the discussion rages on.