Zero is by the Replicache team and they are really nice guys. I've interacted with Aaron Boodman quite a bit in their Discord and he was very responsive and helpful, and took a lot of my comments into consideration in improving the product. It's a very hot space which IMO will revolutionize how we build GUIs IMO. Replicache is a bit hard to grasp at first but the nice thing is you bring your own DB (vs. use a service). I think this project needs more recognition. I would hate to see them fade just because they don't have the fancy backers like YC and the usual crowd. If they keep at it they might prevail since VC-backed startups like InstantDB tend to sell out and flame out.
I think it would be difficult to compare at this point, as there aren't many details about Zero outside of the blog on their landing page. I believe it's very much in early (but active) development.
From that blog:
> We are working toward a source release in summer 2024 and an open beta EOY 2024.
I haven't looked in detail yet — what are the main differences relative to Zero?