I've only heard the "Siri" voice on Apple. It's good but I think I give the edge to Ivona. UK Amy in particular sounds very natural and pleasing to me, more so than the other Ivona voices. Maybe that's because I'm American; maybe it sounds more robotic to a Brit, just like Siri still sounds a little robotic to me.
Does iOS allow third party TTS like Android? Android has had TTS for a while and there's an API to access the service so app support is widespread. On iOS, at least til recently, apps that want to have TTS have to license their own.
Voice quality aside, ivona can impress by plowing through long sentences and difficult words, but it still makes some dumb errors. For example, it insists on reading No. as number when there is no number after. It knows Mr. Smith is mister Smith but pauses after mister, like an end of sentence. It pronounces "crime" with an up beat, like asking a question. I report these to ivona and they thank me but never seem to fix anything.
