If you're choosing between the PSR-S970 and PSR-I455, the PSR-S970 would definitely be a better choice.
The PSR-I models are essentially PSR-E models which have been customized for Indian music; the older PSR-I425 is a customized variation of the PSR-E403, and the newer PSR-I455 is a customized variation of the PSR-E433. If Yamaha sticks to that development and release schedule, it's possible that a new PSR-I485 model could come out in 2018, being a customized variation of the PSR-E463. But that's just pure speculation on my part, as there are no guarantees that a PSR-E463 is being developed for 2018, that Yamaha intends to produce more PSR-I models, or that the next PSR-I model (if there is to be one) will be based on the PSR-E463.
In any case, the PSR-E models are only XGlite-compatible, whereas the PSR-S models are XG-compatible. XGlite is basically a cut-down version of XG (Yamaha's expanded system of MIDI soundsets and messages). There are variations of XGlite depending on how many optional voices, effects, and other features are included on a particular model; but in general XGlite voices are not as rich-sounding as XG voices, and the effects are not as varied. Also, the XGlite models can play only 2 variations of each style (A and B), whereas XG models can play 4 variations (A, B, C, and D). The PSR-S models are XG models, and the PSR-S970 is currently the top of the PSR-S line. Another big difference is that you can buy new voices, styles, and multi pads for the PSR-S970, and you can create new entries for its music database. In contrast, there is no way to add new voices to the PSR-E and PSR-I models; you can add new styles but can load and use (or "register") only a few of them at once; there are no multi pads; and the music database cannot be expanded with new entries. Those aren't the only differences, but they're the major ones.
From what I've seen online, I personally think the PSR-I models are wildly overpriced considering that they're just variations of PSR-E models, which are considerably less expensive. The PSR-S models are much more expensive than the PSR-E models, and the PSR-I models seem to be priced as though they were comparable in features to some of the PSR-S models, which they are not. Presumably the high cost of the PSR-I models is determined by smaller production batches and less demand as compared to the PSR-E models.
I don't want to confuse you by throwing even more choices at you, but the PSR-S970 is the most expensive of the PSR-S models, and if you are considering buying it then you might want to also consider the PSR-A3000. Like the PSR-I models, the PSR-A models are localized customizations for specific world markets-- in this case, the "A" stands for "Arabic" for "Arabian," so as you might imagine these models are specially customized for playing music from the Middle East and surrounding areas. They have buttons for accessing a scale-tuning feature which lets you fine-tune the individual notes of the scale (the PSR-I, PSR-E, and PSR-S models also have that ability, but don't have buttons for accessing that feature as easily). I've also been told that their portamento options are better than those of the PSR-S models for playing Middle Eastern music. So the PSR-A3000 might also be a very good choice for you. I don't know how many Indian voices and styles are built in, but there are Voice & Style Expansions available for Indian music.
EDIT: I might add that I own three PSR-E models myself, a PSR-E433, PSR-E443, and YPT-400 (which is the same as the PSR-E403 that you have but with a different name). I'm happy with them despite their limitations, although of course I'd love to have a PSR-S970 or PSR-A3000 if I could afford one. So I don't mean to disparage the PSR-E or PSR-I models, especially if price is a deciding factor for you. But if the PSR-S970 is within your budget-- or even the less-expensive PSR-S770 model-- then it would be a more powerful, flexible, and expandable choice than the PSR-I455.
