...
- All up front
- Biggest discount
- Full payment is made at the start of the term, with no other costs incurred for the remainder of the term, regardless of hours used.
- Partial up front
- A portion of the cost must be paid upfront and the remaining hours in the term are billed at a discounted hourly rate, regardless of usage.
- No up front
- One year term
- Available with a 3 year term for C4, M4, R4, I3, P2, X1, and T2 Standard Reserved Instances only
- You are billed a discounted hourly rate for every hour within the term, regardless of usage, and no upfront payment is required.
...
- No up front cost purchase type
- Regional
- Standard RI (assuming your needs won't change that much)
Documentation
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-reserved-instances.html