Snapchat is really good, and I think Snapchat Stories and Shorts appeal to users for similar reasons: They're lightweight, low-stress ways to share your life with others, and to feel like you're with your friends throughout the day.
One of the big differences between the two product relates to content creation: On Snapchat you can only create content (for your story) using the in-app camera. On Shorts you can only pull in existing content from your camera roll. Neither approach is better than the other, but the two end up having a very different feel.
More importantly, I don't think it's an issue of using one instead of the other. If you talk to heavy Snapchat users, I think you'll find that they're also heavy users of the native iOS camera app, which means they have a lot of unshared photos on their camera rolls.
Part of the reason for this is that they use different camera apps depending on the context. For example, they might use the native iOS Camera app ...
- When they're getting ready to go out (and the Snapchat camera when they're out, since they're more likely to want to post to their story then)
- If they want to take multiple photos in a row (because it currently takes fewer taps to do that using the native Camera app)
- If they're having someone else take the photo (because currently, more people know how to use the native Camera app than Snapchat)
- If they might want to share the photo to Instagram (because their perception is that the native Camera app takes higher quality photos)
Because (1) it's painful to share camera roll photos, (2) you can't upload camera roll photos to your story, and (3) the norms of existing services don't support high frequency, undirected sharing, most photos taken with the native Camera app stay stuck on our phones, hard drives, and cloud storage services, never to be shared with anyone.
We think this is going to become more of a problem over time as more of our lives are captured on photo, video, or future equivalents.
In other words, Shorts and Snapchat stories both help you share your day with friends, but there are subtle differences that make one more appropriate to use than the other, depending on the context and the moment you want to share.
I think this is something we see with a lot of communication products. Think of the differences between texting, emailing, sending a Facebook message, writing a letter, or sending a PDF document to someone. They're all just sharing text, but so, so, so different. It's a really interesting part of social product design.