Choose Fly.io if you're a solo developer focused on creating latency-sensitive applications. Its ability to deploy across multiple regions ensures low latency for users while the scale-to-zero feature helps manage costs effectively. However, be prepared for unpredictable usage-based billing and a bit of ops knowledge, as it requires more hands-on management than a fully managed platform.
On the other hand, go with Railway if you prioritize ease of use and want to deploy your apps without dealing with infrastructure. Its Git-push simplicity and user-friendly dashboard make it ideal for indie hackers or solo developers looking to quickly iterate. Just keep in mind that after the trial, there’s no permanent free tier, and costs can rise with persistent services, so it's not suitable if you need a predictable billing structure or precise regional control.