Whats the difference between Utau synth and the actual UTAU program? :\
UTAU-Synth is made for Mac users.
PC UTAU doesn't run in Mac (unless using wine or emulator but won't work always 100%, ya know?) That's why it's great that Ameya has made Mac ver of UTAU!

Abilities/things what UTAU-Synth has but PC UTAU doesn't:
- Multitrack
- Auto CV/VCV for JP hiragana VBs (this is in PC UTAU's shareware but not in free ver)
- You can see envelopes which is pretty handy (again, I think that you can see them too in shareware)
- This is little off topic but anyway: This is just MY opinion but I think that interface is prettier in Synth than PC UTAU
What UTAU-Synth lacks but it is in PC UTAU:
- Ability to fix/edit FRQ files
- No different "resamples" or any kind of "plug-ins"
This are main differences what I can thing between Mac and PC UTAU. Of course there's more to add but here are main differences