Mike, 

I am not a TTS developer either, I agree that eSpeak is a fantastic piece of code. As someone who started programming on the ZX81 and Atari 400 I can appreciate compact code. 

The eSpeak voice is intelligible, and so I'm lead to believe, can still be understood at high speeds. 

The eSpeak voice is generally not aesthetically pleasing to those less familiar with TTS. I think Ken was looking to the MaryTTS voices to find something more appealing to the general listener.

Having had a little more time to play with MaryTTS I can now appreciate that although seemingly more natural some elements of the voices are less intelligible at times. This may be the clipping you referred to on the mail list (I don't know because I don't really have a vocabulary to describe TTS voice quality). What I have noticed is that there are two types of MaryTTS voices,  conversely to expectations the ones with the larger data-set appear to be less intelligible. 

Which if any of the Mary voices are the clearest/cleanest?

The goals of eSpeak and MaryTTS are somewhat different, the Mary project appears to be a university research project. Having had a bit of a dig around in the MaryTTS code, I've found that it includes a lot of tools for recording and creating voices. There is also a whole range of effects processing and other tools to amend the vocal output model. Its definitely not a light weight TTS solution, but I don't think that was ever the intention.

I did notice that eSpeak can create static wav files using the -w switch so it probably wins there too.

Where MaryTTS or similar projects may win out over eSpeak would be to provide a more suitable voice to those who rely on speech synthesis to be able to speak. I recently heard this TED talk. https://www.ted.com/talks/rupal_patel_synthetic_voices_as_unique_as_fingerprints . The voices featured here appear to be a great improvement over MaryTTS, but I don't know what software they are using or if it is open source. 