SY55 (which I owned):
-straight sample playback synth
-16 note polyphony (that takes you down to 4 when using 4 elements/waveforms)
-74 16bit/48kHz waveforms in ROM
- 8 track sequencer with 8,000 note capacity
- expansion slots for both RAM/ROM patch cards and Waveform cards.
-onboard EFX processor
-2 multimode filters per element
SY35:
-vector synthesizer (much like Sequential Circuits Prophet VS and
Korg Wavestation)
-256 PCM (12bit) and 2 operator FM synthesis
-32 note polyphony
-no sequencer
-no waveform expansion capabilities, only patches
-onboard EFX processor
- no filters
If you have to pick one, it all depends on what you're looking for. The 35 would be a better choice for true "synth" sounds and not realistic ones.
The 55 has an overall cleaner sound, but some of the waveforms in ROM are rather unimpressive (especially drums).
Looking back, I only bought the 55 because it had the onboard sequencer - if I could do it all over again, I would probably have bought the 35 instead.
If you have the money, I would skip both the 35 and 55 and hunt down
a SY77 or
SY99 - they are both considered classics now.