Linear is, for a lot of indie developers, the most beautiful issue tracker ever built. Cycles, projects, issues, sub-issues, triage — all wrapped in keyboard-first UI and tight Slack/GitHub integration. It's designed for software teams that ship continuously.
Milestones is built for the *single* person planning their own work. Project → Milestone → Task, no cycles, no triage queues, no team admin. It's native iOS, iPadOS, and macOS, and it sync privately through iCloud.
If you're running a small team building a product, Linear is probably the right tool. If you're a solo person who keeps copy-pasting Linear's free tier into yet another team workspace just to track your own things, Milestones is closer to the shape of your problem.