The paperback fiction novels are mostly about 6 inches. But textbooks, or books on serious non-fiction subjects(without images) dont exist in 6 inch format for the most part, theyre much bigger. There's must be some reason behind that. I can't imagine reading a book on public administration on a 6inch kindle, its like reading on a slightly bigger mobile phone. Well, may be its just me.
And hey, if kindle works great for you and ur looking at ipad as an option only for its PDF support , check this great free software called 'calibre e-book management' which converts PDFs to .mobi format, making them perfectly readable on Kindle. You just need to input a pdf and it has a specific 'output' for kindle.
I think i just saved you a few hundered dollars on ipad mini
And i just borrowed an ipad 3 for sometime. Will try reading on it and share my experience