Slide feels more modern, presents images and videos more prominently and overall, it feels like you're consuming more content. It's also a bit buggier and I found using it to be more strenuous, just because you're going through more content.
RedReader has a much more conservative and simpler navigation scheme, feels more text-focused and more reliable. It's missing some features compared to Slide, for example a "gallery view" for image-focused subreddits or some filtering and customization features, but it also has its own strengths.
For me, RedReader's offline-viewing feature worked a lot better. It (pre-)caches posts while you're reading and then you can just transparently read on when you lose internet. Slide seemed to not cache as much and while it has a separate feature to download entire subreddits' posts at a set time, that never seemed to work reliably for me (again seemed to not cache that much).
Another relatively big detail is that in RedReader, you can vote by sliding a post or comment left/right. In Slide, you have to instead hit a button, which is much harder to do and takes you out of the reading flow.
Sliding is then again reserved in Slide for navigation. For example, you generally slide left to right to go back. That works well, too, makes for a fluid feel, but I find I have to often stop and think for half a second what my slide will do or how I have to slide to do what I want to do. This adds to the strenuous feel, but I can imagine that this becomes really intuitive, if you use the app for a while.
Personally, I actually had both installed most of the time and switched back and forth between them (which also creates problems, like filter lists & settings not being shared).
Most of the time, I spent in RedReader, but that was purely subjective. I liked the simplicity and reliability. I do think that more people would prefer Slide. It seems to follow the design that's currently popular with Reddit apps.
Mind that I've stopped using Reddit about two months ago (when they made up obvious bullshit to excuse not anymore open-sourcing the website code), so that's about how up-to-date I am on these two apps.
Slide feels more modern, presents images and videos more prominently and overall, it feels like you're consuming more content. It's also a bit buggier and I found using it to be more strenuous, just because you're going through more content.
RedReader has a much more conservative and simpler navigation scheme, feels more text-focused and more reliable. It's missing some features compared to Slide, for example a "gallery view" for image-focused subreddits or some filtering and customization features, but it also has its own strengths.
For me, RedReader's offline-viewing feature worked a lot better. It (pre-)caches posts while you're reading and then you can just transparently read on when you lose internet. Slide seemed to not cache as much and while it has a separate feature to download entire subreddits' posts at a set time, that never seemed to work reliably for me (again seemed to not cache that much).
Another relatively big detail is that in RedReader, you can vote by sliding a post or comment left/right. In Slide, you have to instead hit a button, which is much harder to do and takes you out of the reading flow.
Sliding is then again reserved in Slide for navigation. For example, you generally slide left to right to go back. That works well, too, makes for a fluid feel, but I find I have to often stop and think for half a second what my slide will do or how I have to slide to do what I want to do. This adds to the strenuous feel, but I can imagine that this becomes really intuitive, if you use the app for a while.
Personally, I actually had both installed most of the time and switched back and forth between them (which also creates problems, like filter lists & settings not being shared).
Most of the time, I spent in RedReader, but that was purely subjective. I liked the simplicity and reliability. I do think that more people would prefer Slide. It seems to follow the design that's currently popular with Reddit apps.
Mind that I've stopped using Reddit about two months ago (when they made up obvious bullshit to excuse not anymore open-sourcing the website code), so that's about how up-to-date I am on these two apps.