Stable and Stark at this point are quite similar. However, Stark is using core templates, which may well change over time, potentially breaking subthemes. Stable has duplicated the core templates which will remain consistent throughout the D8 lifecycle. Unlike in D7, there is never a reason to use Stark as a basetheme. Why is it listed on the Appearance page? And worse, why are users prompted to "Learn how to build a custom theme from Stark..."?
I recommend removing Stark from the /appearance page or at least modifying the help text to express Stable or Classy are better options:
Stark
Intended as a testing platform to view default markup. This should never be subthemed as future core updates may break your theme. Instead, core supplies Classy and Stable which are better options. Numerous contributed base themes exist, as well. Learn how to build a custom themes in the Theming Guide.
Thoughts?