My understanding is that Pop uses Flatpak sparingly, whereas Canonical is pushing Snaps for an increasing amount of software.
That is, IIRC, you get (say) Firefox via Debs in Pop. Canonical wants to install it via Snap.
IMO Flatpak is a great option for a set of desktop software where packaging across distros may be problematic for some reason. I use the Firefox Flatpak on Fedora / RHEL because it doesn't disable the video codecs, for instance. The native package doesn't have some codecs enabled.
Canonical is (AIUI) pushing Snap for desktop and server software. And Canonical is the only source of Snaps, I believe? With Flatpak you get Flathub but AFAIK anybody could set up a repo of Flatpaks.
Flatpaks have some startup time penalty, but it's an order of magnitude better than Snaps. They also have way better disk usage characteristics per installed package than Snaps do (they scale better).
You can use them side-by-side on the same system. Install a dozen of each and take some measurements.
Ubuntu, with the smallest cleanest but richest desktop on Linux -- Xfce -- and neither Snap nor Flatpak. Instead, `deb-get` which finds and installs native DEB packages, configures the repos for you, manages updates and so on.
I was on KDE for a bit, love it on my ultrawide monitor with tiling. But on my laptop Gnome is so sweat. I have a 2nd hand HP ProBook and I swear, for the very first time ever in my Linux life the trackpad feels like my MacBook trackpad. 3 fingers swipe up, overview of windows, another swipe is app grid, swipe left right, move to other desktops. I really find the whole experience very smooth and I enjoy it a lot (all Wayland, on NixOS).
I have suspend/wake working well so far, USB-C charging + screen + all peripherals via 1 USB-C cable, all "media" buttons work. And this is still on 8 GB of ram (soon the 2x32 GB will arrive ;)), I haven't even heard the fans so far! I'm on Teams, camera works, nobody would even know I'm on Linux if it wasn't for my constant evangelizing!
I'm really impressed (yeah, I know, Linux people easily are when it comes to desktops, basically we're impressed if things work).
I did enable AppIndicator. Solaar, NextCloud and Tailscale all really require it to be considered functional. I don't understand why that is not a standard thing (it is on many distros though).
I don't like GNOME. I don't like GNOME accessories; I hate CSD and hamburger menus and that big empty wasted top panel. I want more things vertical, while GNOME is moving its vertical workspace-switcher to horizontal.
The Cosmic tiling is good. That's an improvement. GNOME's window management sucks, and this is better.
I also really don't like systemd-boot and it broke one of my laptops severely.
So, I am happy that it is good for you, but it is not something I'd choose.
And I do agree on the very high amount of pixels wasted in the top panel, expecially on an ultrawide monitor, something MacOS does better. On a laptop screen it's ok for me.
Not yet, but it's fairly simple, I now have 1 week of NixOS experience ;) I installed with the graphical installer, choose Gnome, added some specific things like darkmode and many packages to the configuration.nix. Then I read about Home Manager and put packages under there.
OOTB the Gnome install was as good as can be. It's a nice way to play with Nix, batteries included.
Right now I need to deploy a server used for bioinformatics, and I need Conda... And that is a pain [0], so again I'm on the fence: Deploy Ubuntu or NixOS and persevere... I'm thinking Ubuntu, then perhaps later in the learning curve I go for Nix or perhaps just make a container to use on Nix.
I will use them, at least for a while, when their currently in development Rust desktop env is out. Super curious how that pans out (as traditionally all popular GUI libs are very OO).