Software Is Key
We already mentioned the Galaxy Note's S Pen software as being a key difference between the Note and Galaxy S. There will be some other differences too.
While the Galaxy Note 5's software hasn't been detailed just yet, we're expecting it to run Android 5.1.1 (Android M if we're lucky) that's based on the software on the Galaxy S6. The Galaxy S6's version of Lollipop looks much different than the Galaxy S5's. The Galaxy S5 might never get these Galaxy S6/Note 5 features.
The Galaxy Note 5 will almost certainly get Android M before the Galaxy S5 and it will also feature longer software support. Samsung's Galaxy S5 is already a year old which means that it probably has a year of software support left. The Galaxy Note 5 will have at least two.
If you're planning to hold onto your phone for awhile, you'll want to keep this in mind. Samsung probably won't announce it on stage.