I had a 900 and now have a 910. I think the 900 had a somewhat "warmer" sound that I liked but many owners of that model experienced problems with the display screen, an issue that is expensive to fix and the model is now so old that there is little chance that Yamaha would replace the screen for free as they once did even after the warranty period had ended.
The 910 has more voices (678 vs. 392) and styles (322 vs. 305) than the 900. The 910 can use both the older "SFF1" styles and the current "SFF2" styles and that can be an advantage if you wish to have the widest variety of styles available.
If you use the Music Finder feature you would probably like the fact that the Music Finder on the 910 can use styles that are stored on an attached USB drive. The 900 Music Finder can only use factory styles and styles that are stored in the keyboard's User Memory area.
The 910's User Memory area is almost twice as large as the 900's, but both are rather small (2.4MB vs. 1.4 MB). For this reason it is suggested that you plan on using a USB drive for permanent storage and rely on the User Memory area for temporary storage only regardless of model.
Although both models are discontinued you can still view and compare their specifications:http://usa.yamaha.com/products/musical-instruments/keyboards/arranger_workstations/psr-s900/http://usa.yamaha.com/products/musical-instruments/keyboards/arranger_workstations/psr-s910/
Since production of the 900 stopped when the 910 was introduced it is a certainty that any 900 you obtain will be older than any 910, but actual "wear and tear" on different instruments can vary widely so it would be important to try out the keyboards and also know something about their usage history. Certainly ask about any warranty and return policies that may apply.
All other things being equal, I think the 910 would be the preferred model at this time. You might check on eBay or similar sites to see what prices are current. I would think a 900 from a private seller would be in the $500 - $700USD range and a 910 in the $800 - $1000USD range, depending on condition and any extras that might be included.