Any information available to summarize current main differences between Solidus and Spree?

One day I'll put something on our website...

Any obvious differences you could mention here?

Spree is currently basically just maintained by a single company, Spark. Solidus is managed by two teams: the core team and the stakeholders team. These groups comprise people from various different companies who are invested in the platform. I tend to think this is a strength of Solidus. Feature-wise they are very similar, being that Solidus is a fork and the development of it continued after Spree Commerce got acquired, but since Spark picked Spree up they've kept development at a pretty solid pace again.

They recently announced a SAAS version called Vendo. Does Solidus have the same API-based headless approach?

Solidus has an API that you can build headless on, but our default storefront is a normal Rails-based frontend.

Hmmm, I like the build your own UI approach along with their multi-vendor plugin (along with others). Do plugins/extensions work across the two products?

many extensions were forked and made to work with solidus (see contrib) and it's fairly easy to port them over for the most part, but they won't work out of the box

One difference is the slack response time. I have never had this quick and forthcoming responses in the Spree channels!

haha

I like next Solidus frontend and I hope it will be better developed because I don't like headless😀

We have great success with Solidus running headless and the storefront build with Nuxt. But ymmv. It is definitely lots of work and you need a larger team than going with a server rendered rails frontend. I see room for both in the community and since Solidus can be used for both I see a success story here as well.