Cockroach DB is a full SQL compatible, strongly consistent key-value store. This means you can get the scalability and performance of a key-value store alongside the comfortable SQL query language. You, and all the other developers who work with you who already know SQL, can do pretty much all the things you know and love from SQL like joins, secondary indexes, etc with full ACID compliance. This means that when you read data from the database you always know with 100% certainty you're going to get up to date values (no stale reads).
ScyllaDB is similar in that data is stored in tables with a defined schema, but it uses a different query language, CQL[1], which is often similar to SQL. You can't to joins but you can have secondary indexes. You can store most of the same data types that you know and love from a standard SQL store. Interestingly enough, you get to CHOOSE the level of consistency you get, so you can make your ScyllaDB strongly consistent or choose from an array of eventually consistent options[2]. Most people however go with one of the eventually consistent options, which allow Scylla to be insanely performant and scalable. At the cost of strong consistency, you get an extremely high performance at an almost infinite scale. CockroachDB, while performant and scalable, can't match it here. It stands almost on a tier of it's own in terms of scalability and performance.
So really, the choice is yours based on what you're looking for. I'd choose CockroachDB for my purposes since I'm not storing Apple levels of data and consistency is important to my work, but your specifications and needs may be different.
Ultimately the same foundations of a globally distributed key/value store. Scylla is the Cassandra is a mashup of BigTable/Dynamo wide-column advanced key/value design allowing for very high scalability and availability, but the data model and querying abilities are not as flexible. Scylla does have variable per-query consistency settings but is primarily eventually consistent. Does support BATCH statements which are atomic updates, but no Lightweight-Transactions yet to read-before-write, but does have counters now. It's not quite feature parity with Cassandra but quickly getting there.
CockroachDB also uses a key/value store but puts a postgres-compatible SQL layer on top, derived from the Google Spanner approach, so you can get (almost) all the querying abilities and data modeling of a relational database. They're slated to have JSON datatypes soon that will make it very compelling as a general purpose, highly reliable datastore for all of your core data in multiple regions.
If you want to read about the theoretical underpinnings, Cassandra is derived from the original Dynamo paper[0] from Amazon, and Scylla is a drop-in replacement for Cassandra written in C++ instead of Java. Cockroach follows more closely the Google Spanner[1] approach.
For a more practical summary, compare the architecture overviews of Cassandra[2] and Cockroach[3].
They're pretty different. Scylla is essentially a C++ rewrite of Cassandra, while CockroachDB aims to be a true distributed SQL database. Cassandra and Scylla do not support transactions (or atomicity or strong consistency) or joins.
Cockroach DB is a full SQL compatible, strongly consistent key-value store. This means you can get the scalability and performance of a key-value store alongside the comfortable SQL query language. You, and all the other developers who work with you who already know SQL, can do pretty much all the things you know and love from SQL like joins, secondary indexes, etc with full ACID compliance. This means that when you read data from the database you always know with 100% certainty you're going to get up to date values (no stale reads).
ScyllaDB is similar in that data is stored in tables with a defined schema, but it uses a different query language, CQL[1], which is often similar to SQL. You can't to joins but you can have secondary indexes. You can store most of the same data types that you know and love from a standard SQL store. Interestingly enough, you get to CHOOSE the level of consistency you get, so you can make your ScyllaDB strongly consistent or choose from an array of eventually consistent options[2]. Most people however go with one of the eventually consistent options, which allow Scylla to be insanely performant and scalable. At the cost of strong consistency, you get an extremely high performance at an almost infinite scale. CockroachDB, while performant and scalable, can't match it here. It stands almost on a tier of it's own in terms of scalability and performance.
So really, the choice is yours based on what you're looking for. I'd choose CockroachDB for my purposes since I'm not storing Apple levels of data and consistency is important to my work, but your specifications and needs may be different.
[1] http://docs.scylladb.com/getting-started/ddl/ [2] http://docs.scylladb.com/architecture/architecture-fault-tol...