At the end of the day the actual rates themselves are exactly the same (EC2 savings plan vs. RI). So it's more a question of usage patterns and flexibility. Overall, savings plans are better because they offer more flexibility (though are a bit harder to calculate up-front). In my opinion I think RIs will be depreciated eventually because savings plans are superior in basically every way. RIs assume 24x7 usage and lock you into a specific machine for the term. With SPs you simply commit to a certain amount of spend per hour/month, and then however you use that is up to you. Hence you can have 1 machine on 24x7, or 24 machines on for 1 hour a day - that's up to you. You don't get that flexibility with RIs. Calculating a savings plan can be confusing at first bit it's not really that bad, you just have to change your unit from Total Instances (RIs) to Family Spend per hour per month (SPs). In short: Calculate the total hours you will need for each instance type (eg. M4, M5, M6g, etc) in a given month. This can be a combination of 24x7 instanced and on-demand. Knowing the total hours, calculate the total cost of each instance type, given the savings plan rate. Sum the total monthly spend for the entire family (eg. M4+M5+M6g, etc). Divide that total spend by 730 - this is your hourly spend rate which you will purchase your SPs with. Suppose you commit to $1/hr of M-family usage. Amazon will bill you $730 at the beginning of the month for your commitment. At that point, any usage of M-class machines is credited at the savings plan rate, up to $730. After you hit $730, you will be billed for normal on-demand rates. So really all you have to do is figure out how much you intent to spend in a month across a given family, divide by 730. More on reddit.com

Ignore the other comment. Your best option here is a compute savings plan. It will cover EC2 and Fargate usage. A compute savings plan would get the same discount as a convertible RI, same for a standard RI and an EC2 instance savings plan. In terms of scaling, buy what you need now with a CSP, if you scale up and need more, just buy another one. But make sure the usage is going to be there for at least the amount of time to get to the breakeven point versus on demand. Hope this helps! More on reddit.com

Convertible RIs end up being the same price as Compute Savings Plans, and standard RIs end up being the same price as EC2 Savings Plans. For me, Savings Plans have completely rendedered RIs obsolete, especially the CSP (as opposed to the EC2SP). No instance type or region restrictions -- just spin up your instances, change them, do whatever, and it will be billed at the lower rate. No need to "trade in" convertible instances, etc. For your use case where you know you will be within a single instance family in a single region, you may as well use the EC2SP. You just commit to $x/hr and it doesn't matter if you spin up 16 t3.small or 8 t3.medium or 1 t3.2xlarge... it will just be billed at the lower rate and you can change the instance type at will. More on reddit.com

Reserved Instances are based on the commitment to use an instance at a particular price over a specific period, while Savings Plans are based on the commitment to spend a particular dollar amount per hour over a specific period. More on reddit.com