Solidus is a fork of Spree that was created when Spree was being managed by a different company who ended up abandoning the project. Much of the Spree community rallied around the Solidus fork. Solidus is managed by a core team from various different companies. Spree is managed by a single company, Spark Solutions.
One of the main goals of Solidus was to ensure that the upgrade path for stores running the project is painless. Prior to Solidus, the Spree community had become very fragmented because upgrades were too much work. We've been very successful at that.
We don't have an up-to-date roadmap as much of the development is community-driven, but currently the project is spending funds to complete our new, modernized admin interface.
Also, keep in mind that the Spree "
success stories" are somewhat misleading. A number of them are Solidus stores, not Spree stores. The people running the site have a history of putting Solidus stores on there without permission, claiming they are Spree stores, and dragging their heels when stores have asked to be removed.
The technical differences between the projects are hard to list. The project diverged ten years ago, and while they share much of the same code, many things have changed as well. Solidus has a new admin, new starter frontend, new promotions system, and a great many extension points that ease customization that aren't present in Spree. I'm not really qualified to say what Spree has that we don't, besides a more restrictive license, though. 😉