The fundamental difference is that Terragrunt is a wrapper for Terraform, compared to Terramate, which generates native Terraform code that runs in all kinds of Terraform tooling such as CIs, etc. Also, Terramate adds new concepts such as eg change detection, which is helpful when building pipelines. Terramate may also be used together with Terragrunt.

For details please see https://blog.mineiros.io/introducing-terramate-an-orchestrat... and https://blog.mineiros.io/terramate-and-terragrunt-f27f2ec403...