Firstly, I should mention that I like Ubuntu’s desktop spin quite a bit. There’s nothing wrong with it, it’s a solid choice. I think Canonical is doing a really great job with it. Pop!_OS, to me, features improvements that generally push it ahead of Ubuntu in several categories.
First, what’s the same? They both share the same base, and the same desktop environment. That’s probably common knowledge, but a good place to start. Both Ubuntu and Pop!_OS offer a great GNOME experience.
In regards to what sets Pop!_OS apart from Ubuntu, here are my favorite differences. Note that I’m not going to talk about themes, since that’s obvious and people fixate on that one change too much IMHO.
First, Pop!_OS is semi-rolling. The base distribution follows the same cadence as Ubuntu, and the base is not rolling. But the Pop!_OS repositories are semi-rolling, as System76 often pushes newer applications to the repositories that what’s available in Ubuntu, which means that Pop often has newer apps. This also means that when a new major version of Pop!_OS is released, it’s not actually a new version but more of a “snapshot” - new features are typically pushed to the current LTS and the current non-LTS release, essentially giving them feature-parity.
Also, Pop!_OS has better hardware support overall, and that’s not limited to System76 computers. For example, true hybrid graphics support before Ubuntu had it, and they often build and release their own Nvidia drivers that are generally newer than the drivers that are available in the equivalent Ubuntu release.
I think Pop!_OS has a better desktop/laptop experience than Ubuntu proper as well, for example, a recovery partition is added automatically. It allows you to access live mode (without grabbing a USB flash drive) to test things and even reinstall the entire distribution while keeping your data intact. Basically, the same thing Windows and macOS have been doing for a while (at least in regards to a recovery partition) giving users an experience that’s more akin to what they’d expect on a commercial operating system.
The big downside regarding Pop!_OS in my opinion is how quiet System76 generally are regarding changes. When’s the last time you saw a feature announcement from them? There’s been a few announcements here and there, but you’d be forgiven if you didn’t notice any. That’s the problem - they don’t really let users know about a lot of the awesome features they implement. They stay pretty modest and quiet. For example, I found out about them rolling out hybrid graphics support by seeing a developer mention it in passing in their Mattermost chatroom. No press-release, no blog post, nothing. I just happened to be in the chatroom at the right time. Otherwise, I wouldn’t even know that feature was added unless I saw it on accident. Quite a while later, Jeremy from System76 talked about hybrid graphics support publicly on Twitter, but only due to the new Oryx Pro shipping with that enabled out of the box. The feature was there for quite a while before that, but they’ve made no public mention I’ve been made aware of. Almost all of the big changes in Pop!_OS just happen quietly in the background. I’d prefer more news articles about the things they’re doing. This is part of the problem why some people aren’t aware that the theme is not the only change to set it apart from Ubuntu.
Also, the mindset is different. Sure, Pop!_OS uses Ubuntu as its base right now. But I’ve had a discussion with one of their developers, that may not always be the case. Ubuntu will probably serve as the base for the foreseeable future, but the mindset is that if Canonical ever goes a direction that System76 doesn’t think their users will approve of, they’ll drop Ubuntu as their base and not think twice about it.
I’m curious what you mean by slow updates in Pop, they usually ship newer versions of popular apps than Ubuntu generally does. On my end, I use both Snaps and Flatpaks on all my machines, and since they happily coexist I don’t see that as a barrier. Snap packages aren’t exposed in Pop Shop last I checked, but you can still “snap install” things after adding the snapd package.
As for a working relationship, I don’t work with System76 on Pop!_OS or have a relationship on that level. I’ve thought about donating some time to join the team as a volunteer for a few hours here and there because I thought it would be fun to do so, but I’ve never had the time. But I have visited them twice and I’ve had a number of conversations with them, but overall the knowledge I have isn’t any different than what other people have access to (other than the fact I keep a close eye on what they’re doing for selfish reasons).
I hope that helps.