Thanks Thomas Fotherby for your comment and the website. Didn’t know that it exists. I guess the price difference may not necessary do to only the CPU power. However, when I looked again at AWS’s instances table here (https://aws.amazon.com/ec2/pricing/on-demand/), I found that m5.large is 8 ECUs, and c5.large is 9 ICUs. But the difference is not huge thought or even material. AWS was referring to ECU as a unit to measure integer operations (even though benchmarking is usually done at more dimensions, such as floating point, pipeline misses, depth, caching, etc.).
The main idea is to have some sort of a clear transition from one env. to another when it comes to scalability and resources allocation. We are using mostly t2 instances in our dev env. promoting our services to production becomes tricky since we use different types of instances.
I hope that helps