I've had a Kindle for more than two years. I used it CONSTANTLY, read over 300 books on it...until I got an iPad. Now I never touch it, and I read everything on the iPad. Unless you read in direct sunlight, I personally think the iPad is the better device. And if you read at night, there's no contest; the iPad doesn't require a seperate book light to read in the dark.
The weight's a non issue if you get one of the various iPad stands or pillows. I use a WedgePad, but the Peeramid is also good, and there's any number of alternatives in the Accessories section.
But I would strongly recommend actually handling both before deciding. There's a very big difference between the iPad's screen and e-ink--I mostly prefer the backlit advantages, as does my husband, but as you see here, others adore e-ink. I would also second the rec to wait a bit longer if you want to go Kindle. The price on the current model is likely to drop if Amazon brings out a tablet/nook color competitor shortly, as expected.
And I wouldn't consider a nook. They're bigger, heavier, and Barnes & Noble strikes me as a pretty shaky company to bet your library on.