The answer is that you may want both (or not). Cloudwatch provides measurements at a much lower frequency (5 minutes) than the meters (1 sec).
On the other hand, you may find that adding and managing meters on ephemeral instances may not be that important.
You can also chose not to enable the 'AWS EC2' dashboard which in turn will not enable collection of the cloudwatch metrics for instances that can be gotten via meters.
You can use filtering to select certain metrics to certain dashboards. The choice of course is yours.