Having evaluated both, I'd say Cloud Spanner is quite a bit behind in some regards. It's not that Spanner itself can't do something but that Cloud Spanner hasn't productized some important features like multi-datacenter failovers. It's certainly coming but CosmosDB (aka. DocumentDB) does a lot of this today (and has been for awhile as this is not entirely new).
"Azure Cosmos DB accounts that are configured to use strong consistency cannot associate more than one Azure region with their Azure Cosmos DB account."
I didn't claim cross-dc consistency. I said failover, which is shockingly hard to make many competitors do. The key here is that only one DC can take writes but the failover works transparently with your client (also with clear SLAs).
An interesting thing to point out is the current beta for Cloud Spanner does not have multi-region deployments... and instead, allows you to do single-region deployment in your choice of 3 (not >30) regions:
