Usually an app. They tend do be more consistently reliable and I can organize them more easily into groups in my app drawer. I can do that with PWAs, too, but not PWAs using just any browser I prefer.

For Nostr apps, definitely choose an app if there is one available, since NIP-55 is a much more reliable way to do signing than NIP-46.

If no native app is available, though, I choose PWA over opening it in the web browser, so long as it supports NIP-46. Web browser is for things that only support NIP-07.