The history link gives details, but if you want a quick analogy:
Pharo & Cuis both forked from squeak at roughly the same time
Squeak = Debian
Pharo = Arch
Cuis = Alpine
Cuis has rigorously simplified, so its much easier to explore & understand the system - because there's less of it
Cuis's re-engineering of morphic is really quite brilliant. Everything is hardware accelerated vector graphics. EVEN THE FONTS are drawn that way. And they remain crisp even when zoomed (in or out) & rotated.
Pharo does have some cool stuff though. I'd love to see fuel (object serialization) & soil (a fully oo database) ported to cuis. And the big one - seaside
But its not an either-or situation. 99% 9f what you learn & do in one is applicable to the other. But if you're starting out, I would definitely recommend cuis (not implying that you wouldn't necessarily stick with it)
Is the rendering in Cuis really hardware-accelerated? I agree it looks great but think it's all software-rendered using the CPU.
Is the development experience compatible with modern AI-assisted workflows? Coding agents have drastically reduced the entry barrier for playing with software projects, and I can think of some fun ideas for Smalltalk things I've been curious about.