It's not just that there are more voices, the quality of the PSR3000 voices are better. The sounds in the PSR1500 sound like the PSR2100 from about 4 years ago. The voices in the 3000 are not just better because of the Mega Voices, but also the sample Rom is enhanced with better qualilty voices. The polyphony is also a big difference. The 3000 is 128 vs the 1500 only having 64. Also, you get much more complex sounding styles with the 3000 because of the use of the Mega Guitar and Bass sounds. As a music store owner I can only tell you that I have only sold 2 PSR15000's and dozens and dozens of PSR3000's.
I wish Yamaha would not have used the same owners manuals for both products. When customers saw what they were missing with the 1500, they would come back and trade up for the 3000! (I eventually had to sell the 1500 for less money because it was sold several times and returned!)
My opinion..........buy the 3000!
George Kaye
Kaye's Music Scene
Reseda, California