The idea that the expansions available on iPad is even remotely comparable to a full fledged Windows portable machine like the surface is insane. I've tried them all on Apple devices, from things like the HyperDrive to the Mophie devices for the iPhone. All are a kludge and are inelegant at best.
It's comparing apples and oranges. While the iPad has some expansion capabilities, it's still a gimped experience consisting of using gaudy dongles. It's primarily a consumption device and was never intended to even step foot in the same market as the Surface. Comparing iMovie, iPhoto or GarageBand on iPad to Photoshop, Premier, Vegas, or Ableton, Audition or the like, it's not even a comparison. Even the free Windows apps blow the "i programs" on the iPad out of the water, even something like Audacity vs. GarageBand for iPad.
Surface has HDMI, SD built in, among other things. Surface can run the full versions of any Windows software, be it Photoshop, CAD programs, anything - all with 100% built in support for the very good pressure sensitive stylus that's built in. The two devices can't even be sanely compared usage case wise, and I say that as someone who's owned pretty much every version of the iPad thus far and I use my Mini daily.