Also there's two versions of the Samantha voice. One is 11 mb (downloaded by default) and another is 152mb (have to download from settings). The improvement with the 150mb version is very noticeable
Oration uses OpenAI's text-to-speech system, which I personally found to have superior quality to Apple's (but I also downloaded the upgraded Samantha voice and have been enjoying using it, so I appreciate the recommendation!)