My overall assessment is that Helix has the potential to run away with the Kakoune interested crowd! For a new editor Helix looks shockingly polished!
Here is what I like:
- Tree sitter is integrated into Helix. Kakoune use a lot of regexes for syntax highlighting (except when the syntax highlighting can be done by the LSP). I really like that
- LSP is built into Helix so you don't need to worry about adding extra plugins. It just works! (Worth mentioning: Kakoune has amazing LSP integration via the separate kak-lsp plugin though)
- Helix is built in Rust. Kakoune is built in C++. I've encountered the occasional segfault in Kakoune and I would guess that these would be rarer in Helix simply because Rust makes it easy to write "safer" code by default
I find Kakoune as more minimalist and consistent in its editing philosophy. Helix AFAIK does not have a good plugin story yet. OTOH it is almost trivial to write a plugin in Kakoune.
Kakoune's pure-editing features (multiple cursors, selections and other primitives) seem richer and deeper. Helix has a wider selection of features "out of the box". As Kakoune has been around for a while it feels more mature. Helix is designed to be an "editor for all" and tries to be a bit like spacemacs with the spacebar as an important key. (BTW Kakoune has relegated the large spacebar key to a relatively minor functionality which is a pity).
Helix feels a bit more responsive and snappy than Kakoune on my machine. (But both are very fast). That could be because I was comparing vanilla Helix with Kakoune+plugins. However, long term, because Kakoune likes shelling out frequently, that might be a bottleneck on how responsive Kakoune will always ultimately be.
Helix seems to have a very fast pace of development. Kakoune moves much more slowly and ponderously. Kakoune's creator is quite hesitant about adding new features possibly in a desire to keep the core editor small and consistent. Kakoune also tends to be a bit opinionated about accepting features in general in its code-base.
Coming to the innards of Helix & Kakoune you can see the differences in the philosophy of both editors. The Helix code base liberally uses many rust crates to build the editor. Apart from a recent C++ compiler and a posix system Kakoune has literally _no_ dependencies -- not even ncurses or a regex library! Basically, Kakoune likes to build everything in-house. This approach has its benefits and pitfalls which I'm not going to debate here.
While Kakoune has had a lot of community contributions over the years, Helix seems to have lot of community momentum behind it currently.