The Ketron SD2 is very good for acoustic sounds. I don't know the SD50. I've got the Roland XV-2020: the Ketron is definatally better for acoustic sounds, the Roland is better for electronic or "danse" sounds.
If ou are using GM sounds, these units are really plug and play and don't require a strong computer.


Free tools for Biab:
Biab Scales and BIArpeggio to practice scales/arpeggio
Biab Explorer: the other song picker