Oooh, interesting question. I haven't used a Polysix, but I can say with certainty that there are some happy quirks to the Lambda's operation that would be difficult to get right with a 6-voice polysynth. Not the least of which is full polyphony. The Lambda's characteristics include:
- 3 oscs, each detunable, each with their own different-phase LFO modulation
- layering of presets has unexpected effects on the sound; for example, mixing Percussive and Ensemble sounds thins the Ensemble automatically so it doesn't get too cluttered
- presets are made using nearly-but-not-quite-sawtooth waves, made from layered squares
- presets are made using fixed filter banks instead of your typical poly's 24dB low-pass
- only the Brass preset has its own 12dB low-pass filter with preset resonance
- chorus effect can be added to Percussive and/or Ensemble parts separately
- envelopes and trem/vibrato for each bank are set independently
Bearing in mind you can do all this with no layering/overdubbing and all that, the Lambda makes its kind of thick, creamy, layered pads very easy indeed. The equivalent with a 6-voice would require several takes, and you'd want to pay attention to the way the notes decay with full poly as opposed to 6-voice poly when starting new notes beyond the voice count.
If you're wanting to mimic a stringer, you can do some of the things a stringer does with a regular poly. But not all of it, and certainly not all at once.
Perhaps try your hand at doing a stringer-type thing with the Polysix, and see how it goes. If it's a drag, you know what to do.
Of course, I'd always recommend buying a Lambda anyway, 'cos I think they're great.