My take on this would be... If you already have a Korg M3, why duplicate it with another Korg? There isn't anything in the PA800's voice set that isn't probably better in the M3's, or it's vastly superior sampler can't add.

BUT.... the S900's SA voices and Mega voice technology is completely different to Korg's imitative sounds, and could add (at considerably less cost than a T2) some very realistic sounds that the M3 is NOT capable of doing. The only thing I would double-check is the MIDI implementation of the S900... can it send any keyboard or arranger part out to the M3 and not to it's internal engine, and can it receive on any channel from the M3's arpeggiator?

Integrating those two different soundsets and pattern creation tools into one should make for a very flexible system...

BTW, I can't wait to play an M3, the demos are awesome. My local dealer should have one soon. How is the Karma 2 technology at doing more traditional styles like jazz or pop?