Yew predates Seed, and provided inspiration. At the time, Yew wasn't in a usable state. There were no full examples, no documentation, and the starter code in the Readme didn't coincide with the release. I believe they fixed those sometime after Seed's release.
This doesn't directly answer your question, but is the context in why Seed started, with Yew already existing.
An immediately noticeable difference is that Yew uses a JSX-like syntax, while Seed uses a custom API based on list-like macros.
I used both yew and seed, but the svg support is lacking. Both framework don't render svg elements correctly. As I'm the author of svgbob[0], which heavily uses svg. I wasn't satisfied with both of the frameworks, So I created sauron web framework and I was quite happy with the result. Not only I can do an server side rendering for svgbob[0], I can also write a text-editor[1] with it and achieve a ~15ms typing latency.
This doesn't directly answer your question, but is the context in why Seed started, with Yew already existing.
An immediately noticeable difference is that Yew uses a JSX-like syntax, while Seed uses a custom API based on list-like macros.