Todoist is the everywhere-app of to-do lists. Web, iOS, Android, Windows, Mac, browser extensions, Apple Watch, Wear OS, Outlook plugins — if it has a screen, Todoist runs there. Tasks live in projects with due dates, priorities, labels, and natural-language input that's been refined for over fifteen years.
Milestones is the opposite trade-off: only iOS, iPadOS, and macOS, but truly native on each. The headline structure isn't a flat list of tasks — it's Project → Milestone → Task, with deadlines on milestones and an active-milestone view that tells you what's next.
If you live across multiple operating systems, Todoist is hard to beat. If your devices all have an Apple logo and your projects deserve more structure than a flat list, Milestones is closer to the shape of the work.