TTS:

TTS is more adaptable Because script designers can generate & edit scripts at a faster pace, they can keep up with the changes that inevitably occur.

TTS is faster It allows script designers to write & edit their coursework lessons or call flows & have them up & running immediately. Voice-over usually requires more time to hire a voice-over specialist & record all the necessary script.

A company might need to adjust its product offerings or services to react to market demands & immediately incorporate those changes in to their IVR script. An educator may need to adjust a coursework in light of a used study or expertise adjust (think groundbreaking study that changes the way educators think about a subject or a program update that requires new scripts for a training class).

No need for voice-over talent...Not everyone can do voice-over because it requires a lovely voice with an easy-to-understand accent (the least amount of accent feasible).

Voice-over:

Because of that, most organizations need to hire a professional, which takes money & time, factoring in changes (you need to track down the original voice talent to keep the voice in a script uniform, & you need to re-record whole sections even if youre only changing a few words).

For messaging that requires high inflection & emotions like excitement or empathy (think an educational program for small kids who might become bored with a monotone voice or an IVR (http://www.plumvoice.com/) for a mental health facility), human voice is more natural.

Voice quality Years ago, TTS messages were robotic & monotone, & although the expertise has improved dramatically in the last few years to become more natural (& will continue to do so), it still at this point has a less human feel to it.