The s770 has better drums & 140mb of expandable memory, the T3 has super articulation 2 voices (phrased woodwinds, solo brass), nicer keybed, larger screen. so it depends what you're going for; if you're making beats or might need downloaded sounds, I'd stick with the s770 (and it's also sleeker and lighter)
But if you *must* have phrased articulation, then the T3. But it's a bigger & heavier, not as expandable (currently not at all, as they've removed the samples from Yamaha Music soft), doesn't have the "Real Drum" kit used on most pre-Genos styles, and doesn't have built in speakers, and isn't compatible with YEM.