An other difference is that one can reply to an email but not to a RSS feed. Reading the comments here it seems to me it is a key point relevant to TinyLetter versus Twittbook.
TinyLetter is a one-way communication channel. Like a blog with comments disabled. This is to me a major feature for two reasons that are actually one: spam. With comments gone you don't have to deal with spam, trolls, etc. The second reason is "social interaction spam": when feedback is enabled, people may think they have to reply, support, "like", "retweet" etc. Often the reply is void of content.
TinyLetter's key feature is that you are sending a "newsletter", making it clear that no feedback is required, which eliminates the social spam that pervades social networks.