Welcome to the forums!
I just wanted to let you know that I split your post from the topic where you had posted it (extracting WAV files from PPI) so it will have its own topic that reflects your question, because I was afraid that it wouldn't be seen and replied to by as many people if they thought it was about that other topic.
As for your question, I'm not up on all of the differences between therm, but I think the S770 is pretty close in specs to the S970, so it should be a good alternative to the more expensive S970 if you can't afford the S970. But if you can afford the S970 then it would probably be the better keyboard to buy.