I currently have a MuTronIII and a TruTron3, and I've had the Q-Tron Plus.
The Q-Tron is the most affordable ($170 new) and can pull off the Jer quack quite well. I really liked mine but wanted the real deal.http://www.ehx.com/browse/envelope-filters
Love the vintage MuIII. Had mine reconditioned by Lingenberg sound and it's good as new. Beautiful organic, syrupy deep quack. But, finding one on ebay or craigslist will cost big dollars and will most likely need service to bring it up to spec.
Recently got one of the new TruTron3's and I have to say I really like it. Made by Mike Biegel who designed the original Mu. I was skeptical that it would live up to the vintage version, but it has it's own strengths that the original doesn't have. In fact, I'm going to keep both of them in my rig to be used in different situations...and the GCX switcher accommodates that nicely. e.g. I prefer how the Tru behaves with my earth drive engaged better than the MuIII. But, I like how the Mu rings out when hitting a chord (think Estimated F#m).http://mu-fx-trutron3x.com/
The bottom line is these effects are finicky and you'll probably have to experiment to find what best suits your needs, rig, and budget.
Hope this helps