They are pretty much identical. Seed has better docs, Yew has a promise for multi-threaded stuff, but its not very well documented (or even working). I've tried projects with both and the nice thing with Yew is that the syntax for HTML is kinda compatible with the actual thing (think JSX.) Seed however is macros only. Both seem to compile in the same time, but I would bet there would be differences on bigger projects (which there are none atm?)
Re big projects: I'm (sort of) using Seed on web-based scheduling and training software used by 7 operational fighter squadrons.
The most complicated pages are in React/TS (eg the actual scheduling board), and the Seed pages are/were mostly simpler ones, like tracking qualifications, personalized schedules etc. I'm actively rewriting the smaller pages from Seed/Rust and TS/React to HTML/CSS/JS for performance and simplicity reasons.
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.