They are different models. OpenCode is trying to be a claude code/codex replacement, where-as pi is something you build yourself, kind of trying to be an emacs type thing compared to vs-code. As in emacs it is more common to write your own extensions, where as in vs-code most people just download them.
I keep butting into the question of; why opencode, when you've got codex available? Codex is open source as well, and i can't seem to picture a situation where one would want Opencode over Codex.
As far as I can tell, they tick the same boxes- but one has the support of a big boy model provider.