I'm a retired ESL/EFL teacher with many years of experience teaching English at every level from beginner to advanced TOEFL prep.
I think you'll find many more resources available for language learning for a tablet or smartphone, whether Android or Apple. There are numerous apps and online resources available for both. To the best of my knowledge, there are no e-book dictionaries with audio pronunciation guides and text to speech rendering won't help you much with learning pronunciation. If you go with Android, I don't recommend the Kindle Fire because the app store is much more limited than Google Play.
This article contains some good info about apps for learning English:
http://busyteacher.org/12155-9-best-...-students.html
You can find many more resources with a Google search.
You may also find amazon's Whispersync for Voice to be a useful tool, since it allows you to read an ebook and listen to the audio book at the same time. It's available in Kindle for Android and Kindle for iOS.