I have not demoed the EXR-3 so my comments are based on the EXR-5 (which I have demoed) and the specs.
1. If you are currently using your computer to create music, then a disk drive on a keyboard would not be necessary.
2. My Roland EM20 has 7 watts per channel and my Yamaha PSR550 is 6 watts per channel, and this is adequate. But, the EXR-3 has 5 watts per channel???? hmmm. You want to hear it for yourself, to see if it is adequate.
The voices would be exactly the same. Roland has many good voices. Many Roland voices are superior to the PSR550:
trumpet, trombone, vibes, strings, brass, organs, piano, pan flute. Yamaha excels on the sweet voices. The electric pianos and organs are very good on the EXR-5.
Starkeeper