I've got both, and they've kind of fit into different roles. When I want to just sit down and read a book, Kindle wins. If I want to read while walking on the treadmill at the gym, Kindle wins because I'm not going to take the iPad to the gym.
When I'm doing other things or trying to unwind before bed, iPad wins out. Usually I start out reading some web pages or watching a video then decide I want to keep reading some book. Weight isn't really an issue for me, and if the screen is dimmed or reverse contrast it's not too hard to read.
I like the iBooks app better than the Kindle app, but I like being able to sync between Kindle and iPad. But the Kindle store is awesome on the Kindle, and terrible on the iPad. Also, the dictionary is so slow and cumbersome on the Kindle I often won't bother using it and skip the word. But on the iPad Kindle, there's no dictionary at all! iBooks wins there - super fast interface.
I never even imagined reading at the gym. I swear I'd get some motion sickness reading the Kindle and using a treadmill at the same time.