I would assume that the T4 should be a lot more...
If it were me, it would pretty much come down to what you can afford. If you can afford it, the Tyros line in general is far better than the PSR series, but if not, the PSR S950 sounds like it should give you some good bang for the buck.
Disclaimer: I've only played the T3 and S910 (and I have a PSR 3000), but I never actually played the T4 or the S950.
Good luck either way!