Ask AWS how to cut your bill and you get pointed at three tools with overlapping names and different jobs: Trusted Advisor, Compute Optimizer, and Cost Explorer. Most comparisons list features. The question that actually matters is coverage: of the money you could realistically recover this quarter, how much can each tool even see?
That framing changes the conclusion. The tools are good at what they look at. The problem is the waste categories none of them looks at, which for most accounts include the largest one.
What each tool actually is
Trusted Advisor is a checklist engine. Its cost-optimization category runs point-in-time checks: low-utilization EC2 instances, idle load balancers, unassociated Elastic IPs, underutilized EBS volumes, idle RDS instances, and a handful more. The catch everyone learns at signup: the full cost checks require a Business or Enterprise support plan; the free tier exposes only a sliver. It answers "what known-bad patterns exist right now", one check at a time, with no dollar rollup you can hand to finance.
Compute Optimizer is a rightsizing recommender. It applies ML to utilization history (14 days by default, up to about three months if you enable enhanced infrastructure metrics, which is paid) and names target configurations: this m5.2xlarge should be an m5.large, this gp2 volume should be gp3, this Lambda is over-provisioned on memory, this autoscaling group is oversized. It's free, it's specific, and its blast radius is exactly compute-shaped: EC2, ASGs, EBS, Lambda, some container workloads. It will never tell you about an idle NAT gateway or an unattached volume, because that's not its question.
