The Mini's screen isn't anywhere near as sharp as the current-generation iPad's, but it's lighter and thinner, and has full access to Apple's world-beating App Store. This includes plenty of educational apps (whisper: and games).
This is the Kindle Fire to get. Even more so now, given its recent price drop to $269. The Fire HD 8.9 has direct access to Amazon's considerable cache of books, movies, TV, shows, and music. It also has the most powerful, clearest-sounding speakers of any tablet.
If you can afford to go a bit higher with your budget, the $399 Nexus 10 is the best full-size Android tablet. It also has the sharpest screen of any tablet on the market, and its recent updates have improved performance significantly.
The Paperwhite has that integrated light that Kindle aficionados have been waiting for. It also has a capacitive touch screen rather than the IR-based touch screen found on competing touch-screen models, plus the Paperwhite's screen is a higher-resolution 1,024x768-pixel display with 212 pixels per inch that can render text and images more crisply.
The Nook Simple Touch is a major advancement over the original Nook. In many ways, it's exactly the e-reader we've been waiting for and was hinted at when Sony licensed Neonode's infrared touch technology to finally solve the problem of trying to craft a touch-screen layer on top of an e-ink screen.