In my opinion, any talk of the PSR-S990 is just that-- talk. We have no clue when (or if) the PSR-S990 will be released, or even whether that will be its name.
Fact: The span of time between the releases of the PSR-S970/770 and PSR-S975/775 is approximately the same as the span of time between the releases of the PSR-S950/750 and PSR-S970/770.
Fact: Some people expressed their opinions that the PSR-S975/775 seem to be geared more toward the Asian market, yet they were exhibited at the Winter NAMM and are available now for order from American music stores, expected to be in stock just a few weeks from now.
To me, these two facts suggest that the PSR-S975/775 are not just some "We've tweaked a few things in the PSR-S970/770 to make them more attractive to the Asian music market and are calling those tweaked models the PSR-S975/775" specialized versions which will soon be replaced by the PSR-S990/790 or whatever, but are indeed the replacements for the PSR-S970/770 and will be around for the next 3 years or so.
Of course, that is just speculation on my part, and I may very well be wrong. But I suspect that if you're going to hold your breath waiting for the PSR-S990 (or whatever it's going to be called), then you should probably be prepared to hold your breath for a couple of years. Every new model that comes out can't be some huge advance over the previous model. I know that's what everyone wants, but it doesn't happen that way. If you look at other keyboard lines from Yamaha-- PSR-E, DGX, etc.-- and look at the differences in features from one model to the next, you'll see that sometimes there are very few changes, whereas at other times there are quite a few changes.
If you're happy with your PSR-S970, and don't think the PSR-S975 is different enough, then hang on to your PSR-S970.
If you'd rather have the newest model with whatever new features it has (however few or however many they might be), then get the PSR-S975.