Nightly Builds vs Tilted Online
This page explains the difference between the two versions of Skyrim Together.

Nightly Builds?

This version is abandoned; the code base was completely reworked into a new version called Reborn. As such, the Nightly Builds won't receive any further updates, but you can still play on them until Reborn has caught up to (and replaced) Nightly Builds.
We originally decided to try releasing nightly builds. As they were built off of the latest code every day, there was no guarantee that they would work and were definitely buggy. However, this way players can at least try it out and hopefully see our progress from a new perspective. We do not suggest trying to seriously play the mod like so at the moment; this is just for players who are eager and feel that they can put up with the bugs and crashes associated with code that's still in development.
You can get the nightly builds right now. The client can be found here, and the server’s docker image can be found here. The server is also available in the server folder within the Skyrim game directory once Harbor has updated the client. For a full list of features available in the Nightly Builds, kindly refer to the Nightly Builds features page.
If you've got any further questions about the Nightly Builds, feel free to visit the Nightly Builds FAQ.

Tilted Online / Reborn?

This version is currently unreleased; It will replace the nightly builds as soon as its stable enough.
Tilted Online is the new version of Skyrim Together, which was reworked from the ground up to provide more stability and enables support for other Bethesda games like Fallout 4. Skyrim Together: Reborn refers specifically to the new version of Skyrim Together which is included within Tilted Online, while Tilted Online refers to the overall project (which also includes Fallout Together, for example).
Although Reborn is more advanced than the Nightly Builds in some regards, such as the synchronization of animations or fully syncing dragons, the current version of Reborn is far less stable. For a full list of features completed, in progress, or missing in Tilted Online, visit the Tilted Online features page.
Eventually this version will replace the current version and the Harbor launcher, once we feel like it’s in a better or similar state to the Nightly Builds.