In buying any keyboard there are multiple factors to check.
1. Sound quality
2. tools available
3. convenient operation
4. feel
5. construction
I have a YPG 525 which is a true quality machine. I've had many other keyboards made by Roland, Akai, Casio, Kawai, and Yamaha. Even within a manufacturer's keyboards there are variations in quality.
Yes, the PSR S900 is a wonderful machine. However, like the YPG 525 it has visibility problems in a dark room. Too, though a really wonderful keyboard there is a greater storage capacity for styles and even more features than the PSR S900. If I were in the market for a keyboard I'd have some of the same concerns as you. Like you, finances enter in to the equation and I'd probably buy the PSR S900. I am waiting to see the T3 first before I make any such decision though. From what I hear it's absolutely fabulous and will be unveiled in early October.