In my opinion, I think the N7 and Surface are two very different systems with two different purposes. For me the N7 is a small and light, low cost and very portable solution primarily for fun (reading, music, videos, games, basic web) with some ability for general productivity when needed (email, calender, Evernote). The Surface is essentially to replace an ultrabook to be slightly easier to carry around and a bit cheaper, with a focus on productivity (full office apps, windows applications), however it doesn't have anywhere near the level of portability of the N7.
When I bought my original IPad I wanted it to do both purposes (portable for fun and useful for work) however the IPad is way too limited to run real programs and too large for easy carrying.
I settled on a Nexus 7 for fun and easy portability and a Macbook Air for heavy-duty work (with a windows partition of course!). Two different devices with two different purposes. I think the surface fits in the macbook air category but with lower specs and very limited hard drive space, I am not sure how successful it will be because of this. Looking forward to hearing reviews though.
Just my 2 cents!