Kabinopus,
The reason the drums sounds better in the 950 then the 750 is the addition of the "real drum set" which is from the tyros4 and is used a lot in the 950.
I'm surprised you didn't include in your review the addition in both the 750 and 950 that both include 64MB of memory for expansion packs. Living in Russia there are Eastern Europe, Western Europe, Middle Eastern, etc. expansion packs which add dozens of sampled for these regions sounds and styles. When you tried these keyboards if you hit the expansion button for both styles and voices my guess is that nothing had been previously loaded in.
One other difference is that the 750 can record and playback only .wav files and the 950 can record only .wav but can playback .wav and mp3 files.