It depends on if you wan?t the extra features of the C. Both are great synths and either way you will get the Virus sound. The Classic is actually "just" a B in a new wrapping. Check:
http://www.accessmusic.de/comparisonchart.php4 for the differences. Anyway, a second hand C (in good condition) is still better than a Classic due to the extra features and the more polyphony...