Angular6/MD PWA vs Ionic 3 + Angular 6 PWA
It depends on what you want to achieve.
It looks like PWAs are becoming more and more popular, but they still have some restrictions. For instance, you cannot use geolocation if your PWA app is in the background, and you cannot use local notifications (AFAIK) if you app is in the background as well. If these scenarios are requirements in your project, you probably have to use cordova hybrid app.
You also have limitations on storage when using PWAs. Besides, on iOS PWA support is quite recent and there are still some bugs/unimplemented features
If you want to quickly check what your device can do using only pwa, have a look at this website (using your device's browser)
If you choose to go with PWAs and you don't need your app to look like a native app, then using ionic might not provide such a great benefit to you.
On a side note, as indicated here, you can submit PWAs through app/play stores.