For all its worth who cares about internal speakers , they're all incapable of showing what your keyboard can really do. There's so many choices for outboard systems from super compact to full out I would keep the keyboard if you like it and look in that direction. My Casio PX350 sounds OK with its internal speakers but nothing like the sound from my Carvin practice PA. Just my opinion.
Run the Casio PX350 through Pianoteq and then through a great speaker system. It's breathtaking.
I read somewhere that the speakers are slightly different on the PSR-S970. They sound better to me.
The power consumption of the S970 is 20W vs 16W for the S950. Same power supply though.