Even if they WERE...

Honestly, given the choice, would you chose a semi-weighted 76 or a fully weighted 61 if you HAD to play piano on one or the other?

For me, at least, the semi-weighted 76 gives you the most OPTIONS. If you know full well what you are going to always play every night, choosing a particular type works well. If you don't... You can play just about any style on a semi-76, up to close to full solo piano style. Sure, it's not as good as the full 88, but you've either GOT to lug one of those around 24/7 just in case, if you use a lightweight 61 the rest of the time, or forgo being able to do a full piano part if the mood strikes you.

I don't know about Yamaha's Pianostyle Mode, but Roland's is certainly good enough that, if you want to play a full piano part AND still have the arranger back it, it is capable of tracking your playing (including dynamics) pretty well. I realize there are better ways of doing everything, but they involve carting a lot of gear to achieve it. If you want to cover ALL the bases with the minimum of gear, the semi-76 is possibly the best compromise. Personally, I think that playing organ parts, or flute and sax parts, or even Wurlitzer 200A parts (that have a pretty light and short action compared to a grand) on a fully weighted action no more or less of a compromise than playing grand piano parts on a semi-weighted.

Unless you haul THREE keyboards around (weighted 88, synth weight 61 and a waterfall 61 for organ), you have to make SOME kind of compromise if you need to cover all the bases. If you hone your act around your equipment, you can use anything (I jam out regularly on a 37 note KX-5 no problem - but I have much more restricted choices when I do), but if you prefer to let the MUSIC dictate the equipment rather than the other way around, a semi-76 is the best one keyboard choice, IMO.

I look forward to hearing the S70XS in your hands, though, Ian. Perhaps you can (as an owner) get Yamaha's attention to making the arp section just a BIT more arranger-like, too...?