I use a 64gb class 10 micro sd in my surface for downloads, some videos, other files I need access to. I also have a usb 3 external 500gb hard drive that I only use for system images when I do backups. You can't restore a system image from a micro sd plugged into the surface, can't even save a system image to a micro sd.
For a couple hundred bucks you can have both, and I also found a powered usb 3 hub on
ebay for $15 that lets me have 4 devices attached to my surface when I'm working at home. Works with the docking station great!!!