I've been playing with both and conceptually they're very similar. ECS is a little less sophisticated and extensible than Kubernetes, but that's by design since it only works with AWS services. For example, ECS uses ELB to abstract services whereas Kubernetes has its own service load balancer and service replication mechanism.
I'm a bit new to this, so not quite sure why you say Kuebernetes has its own load balancer. E.g. you can run Kubernetes on AWS. If you do then it uses ELB for load balancing.
I've been playing with both and conceptually they're very similar. ECS is a little less sophisticated and extensible than Kubernetes, but that's by design since it only works with AWS services. For example, ECS uses ELB to abstract services whereas Kubernetes has its own service load balancer and service replication mechanism.