Have done both and I'd say ECS wins almost always, as long as you can be in AWS. If you are true unicorn/hyperscale, Kube will start to pay off. But otherwise, the upgrade treadmill of kube and the blank stare of SWE's who don't want to learn helm charts, argo, ingress controllers etc are not worth dealing with when you can just ship ECS and have it work for years with minimal overhead.

startups/small-medium businesses implementing Kube are doing it for cargo cult reasons or stealth career development/employment security initiatives by the relevant devops or sre teams.

ECS fargate is a must as well because no "server" means a ton less compliance burden.