Both send a push from a script, and both are good at it.
ntfy is open source and free through its public server, with paid plans from $6 a month;
Pushover is a one-time $4.99 per platform. YouLMK is a hosted service at $4 a month that adds the receiving side: a Reach level per source, quiet hours that hold rather than drop, grouping of repeats, up to two actions in the payload, email, Slack and webhook destinations, watches made from a sentence, and an MCP server so an agent can send and watch on its own. If all you need is a push from a script, ntfy or Pushover will do it. If you want rules about what gets through, and something that watches for you, that is what YouLMK is for. Side by side, from their own pages:
ntfy,
Pushover,
Pushcut,
Bark,
Gotify,
changedetection.io,
Visualping,
Healthchecks.io,
Cronitor.