I think the difference is that AlloyDB is basically PostgreSQL where they've modified how it's storing things to offer better performance via better integration

I think the difference is that AlloyDB is basically PostgreSQL where they've modified how it's storing things to offer better performance via better integration with Google's infrastructure - like what Amazon did with AuroraDB. That means that AlloyDB is compatible with all the stuff you're used to using with PostgreSQL. Spanner is a very different database. They've included a PostgreSQL query-language compatibility layer, but that isn't the same as being "100% compatible." You'll still be dealing with some Spanner concepts like not wanting to hot-spot your writes. Spanner does allow really scaling out and a truly distributed database while AlloyDB is an improvement via better integration with Google's infrastructure, but still mostly the same model. Aurora/Alloy do have advantages by decoupling the storage from the server instances including performance and resiliency, but Spanner is really creating a horizontally scalable, distributed database - with some trade-offs.