The iPad versions of the books are better, but still have odd font issues that I don't much like. If they used a reasonable font for the code, it would be great. I'm still deciding which display I like better.
The iPad is more polished, but the Kindle is smaller, cheaper, lighter, and has a battery that lasts for weeks. I keep wanting to touch the Kindle screen, though.
In the end, I prefer reading technical books as PDFs on my laptop, where I can work and read at the same time. The layout is better, and it's nice to switch between code and book quickly.