IMHO the iPad2 and iPhone are meant to be complimentary to each other rather than replace each other.
I bought my iPad 2 mostly for medical use, but I'm finding a WORLD of customized reading content through the iPad that is just amazing. The official Twitter client is great as a way curating customized information/links/articles from individuals or websites of your liking. Also, Flipboard, Zite, and the grandaddy of readers, Instapaper are all very worthy apps. Instapaper is awesome b/c many apps use its API to allow saving articles to read offline. Also Instapaper has a nice editor's picks section.
Then of course, you have the advantage of laptop like functions, games, video, etc with the iPad2. I would highly suggest getting the iPad2 with the intent for mobile internet (either tethered to your Smartphone, or a dedicated 3G iPad) if you are a commuter.
The iPhone, is more of the best of ultimate mobility, and function, but it can be a pain to read on there or watch movies/TV shows for more than 30min.