I have both. Bought a Polyend, sold it, and bought one again 2 months later because it is so versatile, and I got one for cheap.
The main differences are:
- voice count: Synth has less, doesn’t bother me much
- flexibility: Synth has more engines, a greater variety, more parameters, more filter types (the sh4a only has one filter model that is used for all engines, it’s neither a Juno filter nor a Sh101 filter), and works with user samples (granular, regular, and wave tables)
- Effects: Synth has 3, Sh4d has a lot more slots, plus EQ for each channel, plus compressor and master eq, and a much greater variety of algorithms
- UI: both are excellent, knobs are less elegant than encoders for a multitimbral synth
- controllability: Polyend has Macros, and once these can be controlled via Midi, that’s a game changer for a sound engine. Sh4d has macros if you sacrifice one of the 4 modulation slots for each.
- sound: both sound awesome.
Other differences:
- Synth is lighter, has a bigger display, the grid keyboard, and macro knobs
- Sh4a runs on 4 AA batteries.
Backup: Synth gives you access to the storage, you can access all Performances, Patches, Sequences, Wavetables, Samples, copy, paste, rename, delete them as you wish. That’s great, but comes at a price: program changes do not work, because sounds do not have a slot, so there’s no number attached to the sound, they are a file in the file system, and located by name. And that specific file system is limited to 250 entries per directory, so you get a maximum of 250 performances, 250 presets per engine, 500 samples (250 for grains, 250 for DWA), 250 wave tables.
Roland only gives you a blob with all data, so you you can’t buy sounds without losing all you did previously, and the 256 user sounds and 128 patters are all you get.
If you want a playground for lots of different sounds, get the Synth. If you want effects or Polyphony, get the Sh4d. If you can afford it, get both.
FWIW, I use the Synth at home as a sound module for my Cirklon, and I have a small project going with a friend where I bring only my Digitakt (for samples and as as sequencer) and Synth, and that works just fine.