In short, the sx900 has better drums, sample loading options, and piano, choir, guitars, ensemble strings
However, if you're looking for solo voices? The T3's SA2 voices (the 1st yamaha to have them) are excellent; phrased Sax glissandos, sampled pitch bend (instead of artificially using the pitch bend), and even phrased solo violin that behaves different depending how you play.
Most of us would take the sx900; the T3 is only better for folks who want to highlight solo voices in their performances.
Mark