As a developer of one of those apps I've spent some time to collect information regarding the almost 100 features available in text to speech apps on iOS and split those into several segments. Basic features that are expected and available in all apps are not included. The first version of the document was more visual so that one can quickly compare '+' signs and empty space. Now I have made a new version that should be more accessible for visually impaired users as all features are marked with "Yes" and "No".
If you have some suggestions on how to make this comparison better or more accessible or regarding some missing feature in the list, I'll be happy to update the document.
Finally here is the link: https://speechcentral.net/speech-central-vs-voice-dream-reader-vs-speechify/
Hopefully it will help some people to find what is the best text to speech app for them.