Awslogs log driver vs AWS Firelens - which one to use and when

Using Fargate launch type and want to use CloudWatch: You have to use awslog driver in your task definition. You can find more information about the CloudWatch pricing here- https://aws.amazon.com/cloudwatch/pricing/. CloudWatch has a free tier and anything after the free tier cap (metrics, dashboards, alarms, logs, events, etc) has a different pricing calculation. For example, first 10K metrics cost 0.3$ in most regions, but the next 240K will cost 0.1$ while events are priced at 1$ for 1 million

Using Fargate and don't want to use CloudWatch: Use AWS FireLens to push container logs to third party logstorage system. The cost of 3rd party log storage system would come in play here. Data Dog/AppDynamics and others usually offer membership packages (Free/Premium/Enterprise etc). Unlike CloudWatch, each package will give you different capabilities. For example, on the free tier in DataDog you do not have alerts. Also non-AWS native monitoring tools are priced per host/CPU core for a specific amount of hours.

Firelens would also make sense if you want to ship to CloudWatch but want to do upfront filtering versus sending everything to CloudWatch

If you are looking for simplicity and cost-cautious, I would strongly recommend the default AWSlogs log driver.

The default AWSlogs sends logs directly to CloudWatch Logs with no additional costs apart from CloudWatch costs (storage and queries).

FireLens has a slighter more complex setup and has some cost associated with it, and is recommended if you want to direct your container logs to other sources such as ElasticSearch, Kinesis, Datadogs, etc..

Given your question I'd choose the default AWSlogs log driver.