With iOS 16 Apple has made the most notable changes to its built-in text-to-speech voices since their inception in iOS 7. Aside from adding more voices Apple has working on the quality of those voices and now there is a new tier called Premium that matches the quality of best alternative technologies. Everyone can download those voices for free by going to the device Settings and then to Accessibility > Spoken Content > Voices.
Before iOS 16 Speechify had a proposal that appealed to many users: it was offering much higher quality voices than those available in iOS 15 for 139$/year. To make this offer look even more attractive Speechify has disabled access to the higher quality built-in voices in iOS 15 and it does the same to the new Apple’s Premium voices.
Speech Central fully supports the new Premium voices on iOS 16.
As such the biggest difference between Speechify and Speech Central is that only Speech Central can work with new Apple’s iOS 16 Premium quality voices and thanks to that it delivers overall higher quality at the price of 8$ for lifetime use than Speechify does at significantly higher price of 140$/year. Because of this their voice quality is the same, but Speech Central offers much wider range of features as documented in the fact sheet that covers over 100 features in text-to-speech apps.