I have both the Best Buy version of the Surface as well as the Defender. I am a big OB fan, having had the Defender on both my iPhone 4S and my GS3. I bought the case from BB in the interim while waiting on my Defender. Once I received my Defender I was actually a little disappointed that it made the phone too big for me. It added too much weight, thickness and width to the point that it felt uncomfortable to hold and on my belt. The Surface is barely noticeable on my belt and it barely adds any thickness or width. The other issue with the Defender, IMO, is the space above the screen made it a bit more difficult to write with the S-Pen.
The obvious downside of the Surface is that it will not protect as well as the Defender and the soft touch plastic case is too slick. The Defender has the nice tactile feel and will protect the phone from a nuclear event, almost.
I haven't returned my Defender yet and I will probably give it another try for a couple days, but I went back to the Surface after only two days with the Defender. I think that it is very much personal preference here, mainly because of the size of the phone. It you want to try the Surface and you have a BB near you, go try to Platinum case / holster. It's only $34.95 which is cheaper than Seidio. It's the same case, just rebranded for BB.