I have had EMS and Roland vocoders. The Synth is so much better than the Roland vocoders at Vocoding and equal to the EMS. I'd love to try a Sennheiser but I doubt that will ever happen!
In any case, if you find a Synton, grab it. Wendy Carlos used Synton Vocoders I believe.
They also sold them as Dynacord vocoders.