Choose Fly.io if you are a solo developer or working on latency-sensitive applications that require low-latency deployments close to your user base. Its scale-to-zero machines are excellent for keeping costs down when your app isn't in use, but be prepared for the complexities of usage-based billing and managing more operational tasks than a fully managed option.
Opt for LocalStack if you're focused on developing and testing cloud applications in a local environment without incurring cloud costs. It's ideal for instant feedback during development but is limited to specific AWS services. Avoid it if you're looking for an end-to-end managed solution or prefer open-source alternatives.