I wouldn't say that ePub is better than Mobi. Both formats are great for reading.
A couple of questions that might help us help you...
1. Where are you located? Some readers are not sold worldwide, ie. Nook.
2. How did you like the Kindle Keyboard and the Kindle 4 NT? You mentioned that you tried the Kindle Keyboard but don't mention how you liked it, if you had problems, etc.