[Klotar]

Yes, the posters above have pretty much covered it.

Also, in general but not always, the ringers are louder than the notifiers; because ringers are well, ringers.

While it's true that a notifier makes for a cool ringtone, it might not be as suitable as a ringer would be because notifiers are meant to be just that.

And as others have suggested, if you don't like the stock ringtones, you can download others, and/or use a 3rd party sound gain application and increase the gain on your ringtone(s).