I have an iPad 3G 32GB and a Macbook Air 11" Ultimate. The Macbook Air I got five days ago, so the novelty is still there
As I am writing this, I am sitting on the sofa and using my Macbook Air.
If you want to just surf the Internet (with no flash) and check emails, then get the iPad. If you want more than that get the Macbook Air. It is an amazing laptop that is so small, you can carry it almost everywhere. My main computer is a Mac Pro, so I really need one device as a companion to the Mac Pro. It has to be mobile, fast and it shouldn't make me feel limited in any way.
I own iPads since last June. My first one was an iPad 16GB Wifi only model, which I sold in December in order to get the 3G 32 model. If you are planning to have lot of games or movies get the 32GB model and not the 16GB.
My main issue with the iPad is believe or not multitasking! The RAM is too small and when surfing with Safari you will see that. Having more than two tabs causes almost certainly the pages to refresh every time you go back to them. I hate that. I don't want to have the pages get refreshed every time I go back to them. I understand why this is happening but it irritates me nevertheless. Having a touch based GUI is certainly great and the feeling you get surfing the Internet is also nice, but after a couple of months and when the novelty wears off you will start seeing the limitations imposed by the hardware. Another point of criticism is app switching. Apps do tend to resume slowly and there is almost always a lag when apps resume, which I also don't like.
Now that I think about it there is also something else. The keyboard has an annoying auto correction (which can be turned off), the same as on the iPhone. If you don't mind it, you won't have any problems with it.
Now to the Macbook Air. This is an amazing laptop. I got the ultimate configuration with 1.6Ghz CPU, 4GB RAM and 128GB Flash storage. I use it for Internet surfing, Emails, Note taking, movie watching, chatting, video conferencing and many other typical computing tasks. Nothing processor intensive here. The system behaves very fast and you won't notice any lag.
I am really torn apart as I know that having both devices isn't really logical, but if I was to give up one of them it would be the iPad and not the Macbook Air.