As someone who has used TabletPCs since 2003 (not the "toy" over grown Phone OS Tablets) the pen isn't a gimmick, I have OneNote Notebooks, all filled with digital ink that are filly searchable dating back over 10 years, I've completely given up my dependency on paper notebooks and 99% of printed books.If you're just using a device for surfing the web, you should maybe look into a chrome book. They are cheap and will do what you need.
Otherwise, the MacBook Air is far superior. I mean, the pen is a gimmick in a sense. I dunno, seems like Microsoft picked up a pad of paper and pen and wrote down everything they could think of and crammed it into the surface the surface. Why do 20 things poorly when you can do 10 things perfectly like the MacBook Air achieves?
I'd say go with the more superior machine. The Air!
The SP3 is the best implementation of the TabletPC I've ever used and I could never go back to using a clamshell. The SP3 is the perfect size and weight and in portrait mode has the same dimensions as a legal pad. For a Knowledge Worker, Scientist, IT Professional, or Student who wants a one device solution that is highly mobile, I can't think of a better device. The MBA in all honesty is an Ultrabook with 2012 Specs....