Kindle all the way - Mrs SD has one too.
We are big readers (Mrs SD easily romps through 3-4 books a week) and we love the ease of use of the whole Kindle experience - go online, find a book, download in seconds and away you go. No more trudging off to town, leafing through books in a bookshop, queuing up etc and then trudging home. Brilliant!
We also like the variable font sizing - I loathe large font (and have not bought books in the past because of this) and prefer small/dense whereas Mrs SD seems to prefer (despite having the better eyesight of the two of us) about three words to a page.
I also have a preference for longer reads and generally look for books of 4/500 pages and upwards. These tend to be 'heavy' and awkward to hold/balance for long periods of time esp' eg when reading in bed. The Kindle is small, light and tough enough to take the many bounces that occur when I nod off and it falls to the floor.
The Kindle is cheaper too (once you have defrayed the upfront purchase cost which is not much anyway) - Mrs SD reads more than a few Freebies and is a Daily Deals addict and these are cheap as chips too; and they are not rubbish clearance lines books either. Saves us lots of money,
Unless you need a coffee table book or something with heaps of lovely pictures in I cannot see what advantage a traditional book holds over a Kindle apart from the (slightly paranoid imo) less of a theft risk of the papyrus-thingy.