The older VC6 has the old SSM2017 mic amp, 3 band EQ with mid band sweep, compressor, and a 1/4" DI on the front. It was very noisey, but sounded cool.
The new threeQ has a Burr Brown front end, 3 band EQ with mid band sweep, compressor that does much more and is much faster, switchable outputs at +4/-10, has no noise, and you can use the line input in the back as a 1/4" DI.
The new sixQ is some 90 days out, so if you can wait, and spend $350.00, then the new sixQ is the way to go as you get even more features 3 band EQ with mid band sweep, LF sweep, and HF between two frequencies. Plus digital out, gain make up, DI, more metering and switchable transformer mic pre.
If the decision is the old VC6, or the new threeQ, I can tell you the threeQ is a much better unit. IMHO... So I would get the threeQ based on those two units. Just call me to give you some pointers about the new compressor as it works differently than the old one, and a little goes a long way...