How about putting a pair of mustache bars on your 520? I switched out the drops on my Surly for a pair of mustache bars, particularly because of my needs for commuting.

If you are going to buy a separate bike just for commuting, I'd definitely go with a hybrid over a mtb. (I used a Kona Dew for a while, then a Kona Smoke briefly until my son wanted that bike, now a Surly Cross Check.)