Amazon ELB is the more common of the two in our index: it runs on 464 websites against 32 for Application Request Routing. Counts come from real scans, not vendor claims.
AWS ELB is a network load balancer service provided by Amazon Web Services for distributing traffic across multiple targets, such as Amazon EC2 instances, containers, IP addresses, and Lambda functions.
Application Request Routing (ARR) is an extension to Internet Information Server (IIS), which enables an IIS server to function as a load balancer.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.