Both these are approximately in the same price range. However, soundwise, I think the Pa600 is essentially having (a past TOTL)Pa2x sound engine and its the S950(and not the S750)which is sonically closer(perhaps even better than) to Yamaha TOTL Tyros3.
Like most have stated earlier, I too feel , 750(or used 910) plus Pa600 would be a great combo.