There are two cases where developers would choose EI over Inf1 instances: (1) if you need different CPU and memory sizes than what Inf1 offers, then you can use EI to attach acceleration to the EC2 instance with the right mix of CPU and memory for your application (2) if your performance requirements are significantly lower than what the smallest Inf1 instance provides, then using EI could be a more cost effective choice. For example, if you only need 5 TOPS, enough for processing up to 6 concurrent video streams, then using the smallest slice of EI with a C5.large instance could be up to 50% cheaper than using the smallest size of an Inf1 instance.