I have both the Kindle 2 and the iPad and use both for reading Amazon content.

The Kindle is better for outdoors when traveling. It's light, the battery lasts a long time and you can read it on the beach.

The iPad is better for reading in bed at night. With the Kindle I have to have the lights on or use a booklight, which is a pain. The iPad is backlit so you can easily read it in the dark. It feels much more heavy/delicate so I can't imagine using it elsewhere.

I have been taking the iPad along on trips to watch movies and read at night in the hotel room. I usually fall asleep long before I get fatigue, but I never read much more than an hour at a time. On the Kindle I've spent much of a day reading on the beach.

The iPad has a newspaper application called "PressReader" that is freaking awesome. You can download newspapers in their original format and read them offline and it's beautiful. The newspapers aren't free, but very reasonably priced and is great when you want to download a few before a flight to read. I read the SF Chronicle, WSJ, USAToday (McNews), NY Post (fun). It's one of those apps that really makes the iPad shine.

I subscribe to the New Yorker and the NYT updates, which gives you the headlines every few hours.

Portability - Kindle wins
Presentation - iPad wins