Virus B/KB/Indigo has:
- 24 voice polyphony
- No equalizer
- No Moog filters
- 3 MOD matrix sources
- 6 MOD matrix destinations
Virus C/KC/Indigo 2 has:
- More polyphony
- Equalizer
- Moog filters (1, 2, 3 or 4 pole)
- 6 MOD matrix sources
- 9 MOD matrix destinations
They sound the same (except if using the Moog filters and/or the EQ) but of course you can use more parts, use/abuse Unison more and you can make more wicked sounds with the extended MOD matrix with the C. The moog filters are nice too and EQ helps beefing up some sounds...
Also check this comparison chart:
http://accessmusic.de/comparisonchart.php4