On terraform/cloudformation documentation there are two different resources to create an ElastiCache Redis instance:

1. `aws_elasticache_cluster` (<https://www.terraform.io/docs/providers/aws/r/elasticache_cluster.html>)
2. `aws_elasticache_replication_group` (<https://www.terraform.io/docs/providers/aws/r/elasticache_replication_group.html>)

What is the difference between these two? Which one should I choose?