It really comes down to what you already have and how you get your movies / tv shows that you want to watch.
Personally I have an iPhone, iPad and MBA 11". My wife the same, but a MBA 13"...
iPhone = phone calls, SMS, email on the go as well as some games when I'm just hanging about.
iPad = books, games, some movies, music when reading books and more games.
MBA = Everything else, surfing, programming, music, movies, administration, photo editing, storage etc.
But in saying that I do need a computer to re-encode the movies that go into iTunes and end up on the iPad and I don't use the MBA for those duties generally.
An example is a recent OS trip we took. On the plane I used the iPad for games, movies and reading (10hr flight), I hate trying to read on the iPhone or watch movies on it. When on holidays waiting for trolleys, shuttles, I (or more my Son) used the iPhone to play games. When in the hotel room I would download photos, edit them, store them and also browse the inter-webs on the MBA...
So it comes down to if you already have a PC / Mac that you can use at home to get the movies etc into iTunes and onto the iPad, can put up without flash enabled websites for surfing, go the iPad2, if not go the MBA. Just my thoughts on the matter, each device is for something different in my books.