We get asked why do we use one over the other, I see a lot of discussions made around this topic. I can say there is no definitive answer, it all depends on your internal strategy and human capabilities.

So, I hear you say, are there capabilities that puts one ahead of the other?

Azure Resource Manager (ARM) vs. Terraform templates:

1.  ARM is a declarative language as in it submits the entire “goal state” to ARM for deployment – but doesn’t store a state of your infrastructure deployment like Terraform does
2.  ARM template syntax is specific to Azure – not portable – Resources can be deployed to AWS, GCP and Azure alike using Terraform
3.  Azure resources released in ARM template first – Terraform catches on later
4.  Terraform plan checks the deployment behaviour before it deploys and displays any changes before it actually happen – ARM can wipe out infrastructure as quickly as it can create them – what I want to say here – have change control in place to avoid this happening to you 🙂
5.  Some people find Terraform easier to read and digest especially if those templates are moved between developers – Terraform gives you the flexibility of adding comments while this isn’t available in ARM
6.  Both languages lets you embed custom scripts (i.e. in other languages such as Bash, Powershell etc.) in your templates which gives Terraform the advantage when a resource API provider is not yet available in Terraform
7.  Terraform runs pre-flight checks which means you can avoid failed deployments – ARM doesn’t warn you and your deployment might fail half way through deployment

This is my unbiased take on the matter … at the end of the day a clear business strategy should drive those concerns around deployment language and pattern. DevOps mindset has changed, people need to adapt as technologies change and evolve.