The Virus A/B/C were the original VA Viruses. The TI series introduced much more powerful DSPs, all the wavetable stuff and probably other stuff. More in the SoS review. Every iteration of the Virus has it’s fans - I don’t think Access has ever produced a bad synth.
The TI series has far more polyphony than the A/B/C. If you don’t care about wavetables and don’t care about running multiple patches in parallel then the A/B/C are a good choice if you can find them.