Speaking for me, always MOBI, cause most PDF's look shitty on my Kindle. MOBI can always be scaled on a font basis, while PDFs are enlarged as whole pages making them bigger than the screen. Not sure how this behaves on bigger devices like iPad Air etc.... Besides that: Technical books are not meant for E-Readers. E.g: I regret buying "Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp" for Kindle, cause the code snippets are hardly readable and technical books are meant being read (sometimes in a non stringent order) and worked thru, with jumping back and forth, completely different like novels.