I'd probably choose blindness, as from what I understand losing one of your senses usually enhances the others, so it might end up being a somewhat fair trade off.
Then again, there are some things that I would really miss seeing.
Blindness definitely. With down syndrome all of my senses would be hindered instead of just one. You should just say mental retardation because you can't really aquire down syndrome you are born with it. I would miss being able to play video games but I would still have music and I wouldn't need as much assistance as I would if I were mentally challenged.