I've used both the iPad and the Kindle and much, much prefer the iPad for pretty much all of the reasons above and also because, well, the buttons and all that on the Kindle and the seeming acres of plastic. It is way lighter than the iPad, which can be good, but it also feels like it couldn't stand too much abuse. I've already managed to bang the iPad around more than I meant to and it's come through with flying colors.
That said, the next time I'm lucky enough to spend a few weeks in an Outer Banks beach house I'll seriously consider picking up the cheap Kindle to use there. I tried the iPad on the beach a few times in August, but the glare was killing me and the thought of having that machine around all of that sun and salt and sand and beer and suntan lotions and etc ..... well, that was just freaking me out too much to enjoy it.