Secureblue has been my main os in this last year. Reading and carefully checking info, seems clear how Kicksecure is less hardened and safe in comparison.
Secureblue Kicksecure
Fedora modern, fast updates vs Debian more conservative with updates
Sandoxed apps + Flatseal vs Apt (optional Flatpak)
Hardened_malloc vs Basic Glibc malloc
SeLinux vs AppArmor more basic
Wayland forced vs X11 still supported despite of known safety vulns.
New kernels vs Older more stable ones
Modern Clang/LLVM hardening vs Classic mitigations (fortify_source, pie, relro…)
Immutable vs Mutable
Speaking of Secureblue stability:
Being an immutable/atomic os, I always have a previous version. This dramatically reduce any possibility to have my os broken without the possibility to do anything.
The rebase command is amazing to easily switch to another version or fedora immutable os and also window manager. A lot of fun with this.
I never had any broken os installation in this last year but some devs that are trying new Fedora betas exponentially rise this risk (mitigated by the immutable nature).
Speaking of Secureblue usability:
Some apps will not work if downloaded from flatpak! 90% of the times because not compatible with Wayland (so using unsafe X11, Xwayland) or because of Hardened_malloc.
Said this, it takes zero seconds to check if this app works or Wayland so we prefer an alternative one (ex. Onlyoffice vs LibreOffice) or to enable Xwayland with one command (check Secureblue FAQ). As it takes zero second to remove Hardened_malloc on Flatseal for this app.