The S900 is better for drums and basses. It sounds more precise/defined, the S950 is a little more jittery. The difference in sample rate itself is meaningless from a practical standpoint.

The trigger interface is useless, timing is worse than via MIDI, and it actually is serial just like MIDI.

The S950 can save to HD disks, a big plus. The built in OS supports a filter envelope (the actual filter hardware is the same on both, analog switched-capacistance filters, no resonance).

Both transpose via changing the individual voices DACs sampling frequency. Sounds much better than algorithmic transposition.

The S1000 sounds great, but totally different.