[hendzen]

Simple, CockroachDB is a CP system, while Riak is an AP system.

If you need multi-key ACID transactions, and can tolerate potential downtime in the event that some partition loses a majority of its Raft replicas, you might want to use CockroachDB.

If high availability is a concern, and you can tolerate the occasional data conflict in the case of incomparable vector clocks due to writes accepted during a network partition, or, if your schema can be modeled with CRDTs (LWW register, PN counter, Union-Set, etc), you might want to use Riak.