Though they sound better, I'd advise against mbrola, as it's not free, it's closed source, not worth it in my mind...
Yeah, I do a lotta messing with espeak and festival. Generally festival produces a much more intelligible output and better spoken. Its languages/voices are also quite good. So generally, as far as speech clarity is concerned etc., festival is a better alternative. However it is much heavier than espeak, that's why usually espeak gets chosen as a frontend.