Having owned arranger keyboards with semi-weighted keys, and an electric piano with fully weighted keys, I wouldn't even consider anything other than the lightweight touch of the PSR-S950 and other PSR keyboards. When you play every day for a living, 3 to 4 hours a day, 5 to 7 days a week, your hands and arms will thank you for those lighter keys. And lets be real, it's not the keys that make the difference - it's the person playing those keys. Key weight, IMO, has absolutely no bearing on musicianship. Now I know this will stir the usual comments about expression, but this is just my opinion and opinions vary.
Joe Ayala, who recently passed away and was a very good friend of mine, is an incredible piano player, owned a Yamaha Baby Grand, but when he went to work, even in places that had a piano, he played his S-950, and those who were here for the Synthzone Jam a couple years ago, know just how well Joe played and sang. He was among the best I ever heard.
Cheers,
Gary