Amazing...I just went through the same struggle and ended up picking the P80. In fact I actually bought and took home the S80, then returned it for the P80 (plus about 600 bucks back in my pocket). But it wasn't a financial decision.
The really subtle, natural piano feeling and sound of the P80 are just tooooo perfect for pianists. They've really nailed the touch on this one. In my humble opinion - and I tried em all over the past 3 months - at only $999 the P80 is the premier classical / jazz pianists keyboard to be had.
Now I can't address your main concern - sorry to say I never had the piano plug-in with the S80. But I've seen a lot of people give it a kinda shrug of the shoulders as no great improvement over the already very beautiful piano sounds resident in the S80. Just more good pianos.
Let me tell you the main reason I dumped the S80. The tiny LCD display drove me NUTS. Every audio and MIDI term is reduced to 2 or 3 letter abbreviations and there are a lot of sub-screens that must be excavated (with multiple button-dial combinations) to finally get at what you want. If you are quite experienced and agile with MIDI-digital/audio I'm sure its a great synth. But if you are not, the owners manual is USELESS and a real BEAR. Pages and pages about what the synth's functions are and skimpy, frustrating references about HOW to actually IMPLEMENT them step-by-step. NOT a synth for someone (like me) who has only been into MIDI for about a year now - and I've been working in a college MIDI lab regularly with a lot of gear so I'm not a "newbie" either.
Finally...I hate the way you cant really learn important, basic things about these synths til you buy them and FINALLY have an owners manual. (Hats off to the manufacturers who are putting pdf versions of owners manuals on their websites.) With regard to the P80 manual, I find that it gives you easy access to all the basic MIDI functions -especially the all important "local off" so it can be used as a true controller with other modules. Except for the lack of a MIDI thru (not insurmountable either) I'm convinced it can be a fine controller and I'm very happy to be discovering new things about it in the manual. Just one example - reverb is easily scalable on a 1 to 20 scale - you arent just stuck with the room-hall1-hall2-stage scale. And on and on.
Well...there's a million stories in the naked city and that's mine. I'm sure the S80 is a great device too - but not for me.