Can’t speak specifically to anything about nest, and of course this forum is for users of SmartThings. And SmartThings also has an official partnership with skybell and not with ring.
Different people like different devices for different reasons. Some community members like ring better, some like Skybell better, some like Kuna.
If I had to guess just from a partnership standpoint, the fact that skybell lets you view video on demand and with ring you didn’t used to be able to do that at all and now I think you can only do it if you’re paying a monthly fee means that skybell may fit better into the feature set that most DIY home automation customers are looking for. The whole point of integrating with a video doorbell is so that you could have different events trigger it and be triggered by it.
Here’s the Ring IFTTT channel:
And here’s the Skybell IFTTT Channel:
So Looked at just on paper, it’s pretty clear that Skybell would be the more attractive Integration partner for an HA system. Being able to trigger the camera to start recording is huge.
All of that said, I have no idea how the two devices compare from an engineering standpoint, I haven’t looked at either closely. But here is some community discussion: