virus A has less bits than a B , virus C has more bits than a B , virus classic is a B with a new paint job
http://www.access-music.de/comparisonchart.php4
I'd go for a B or classic as they are the best pricewise.
Ive seen virus B's for £230 and virus C's for £500 yet apart from a couple of different filters, slightly more poly and (I think) an EQ section, there isnt much between them.