I first started using Debian with 1.3 and even admined a substantial number (hundreds) of Debian systems in the early part of my career. I still find it very comfortable.

Up until a few years ago I'd never even looked at Arch, when I got into it, I went pretty deep and mostly used it for a year or so.

Then recently I tried going back to Debian for a while. I didn't even bother with stable, and installed a testing snapshot, because I like new software and Arch hooked me on rolling releases.

I was doing all this on a quite new Thinkpad which required some tweaks to get things working. I found myself ending up on the Arch Wiki over and over. The Debian wiki does have some good Debian specific stuff, but it's also full of really old and maybe outdated information.

And then Debian didn't have some needed audio firmware packaged, and Arch did, so I made the switch back to Arch. I'd at first resisted it, because Arch has such a higher upfront cost to get installed and going. Especially with my more complex btrfs on LUKS2 (no LVM) setup. (I also had this working on Debian, it took a bit of doing).

And now I'm probably going to stick with Arch (on my laptops), though I remain wistful about Debian. It was my first real Linux experience and a lot of my early career.

The things that strike me about Arch vs Debian:

They have comparable numbers of packages available, and if you count AUR, Arch has basically everything. But Arch has FAR fewer people working on it.
By my count Arch has:
57 trusted users (they do packages in the community repo) https://www.archlinux.org/people/trusted-users/
28 develpers (they do core and extra package repos, and other things) https://www.archlinux.org/people/developers/
85 people in total

Debian's has:
1434 people on their contributor list for 2020 contributions: https://contributors.debian.org/ these can include bug reports and things.
978 uploading developers: https://nm.debian.org/public/stats/

So Debian has over 10x more people working on the project.

Arch is generally more up to date than Debian testing and unstable (a lot more than stable, but that's obvious) but not by a ton. Generally on Arch for popular packages you'll see a new version show up within 24-48 hours and you'll be rebooting into new kernels very often.
Latest kernel.org stable kernel is 5.9.6
Arch is on 5.9.4 https://tracker.debian.org/pkg/linux
Debian is on 5.9.1 https://tracker.debian.org/pkg/linux

I don't know the specifics, but Debian is probably even more conservative with the Kernel than they are with other things. That said, you can also run LTS kernels on Arch.

Arch rarely changes upstream defaults, so sometimes you have to tweak and configure things. Debian generally has sensible defaults and takes care of things for you, but does modify upstream more frequently.

I find apt, apt-get and dpkg to be a bit more intuitive to use. But under the hood Debian's package management system is a lot more complex than Arch's relatively simple PKGBUILD.

My last point, and something I've been pondering, is the results from their different philosophies. As an outsider observing and using the results of each communities efforts, these are some things I think.

Arch has a pretty extreme view on simplicity. But because of that, they have a rich, and fast moving ecosystem, with many many packages available.

Some of those choices and results:

rolling release - don't bother with releases, freezes, promoting from one branch to another, etc. This saves a TON of effort, and for desktop / personal use, works great. It's by far what I prefer. Even when I was using Debian, I just would run testing or unstable anyway, so would be rolling there too. I haven't come up with a non-insane way to make this work for production systems, but still pondering that.

no real installer - arch just gives you a boot image that basically lets you manually partition your drive and then some tools (pacstrap, arch-chroot) to get things setup and going. This is radically more simple, this also saves a ton of effort for the developers. So much less to test and maintain.

avoid modifying upstream - Also a massive time saver. Just package up what upstream provides and constantly track that.

wiki instead of doing any real setup or defaults - this is also a huge time saver for the developers. Just let people document how to do things and provide the most bare building blocks. Most arch packages just put files in place, and leave all the rest up to the user.

don't split packages - Arch generally doesn't split packages out like Debian does with the -dev packages, etc. This seems simpler and easier to deal with to me. At the cost of a bit more disk space for users.

All that said, everything I've listed above basically offloads a ton of work to each individual user. That may be the fundamental trade-off here. Debian tries to do a lot more for it's users. But they need 10x more developers to do it.