I know only a bit about Synapse. It seems like they have some valuable data. I think the biggest differences are in the culture and user experience. Quilt gets its DNA from the open/community-driven cultures of GitHub and npm. We want to make it extremely light-weight for anyone to push and share data packages. Synapse frankly seems to have a lot more features. We want to compete on simplicity.
Does Synapse offer server-side filtering? That's something we're thinking about.
Does Synapse offer server-side filtering? That's something we're thinking about.