I would go with any mountain derailleur you can find. I know that shifting between small- and middle-chainrings is lightning fast and consistent, and the only compromises in set-up result from the need to shift past the middle ring to get to the big ring. Any front derailleur can be set to slam back and forth between two positions very reliably.
It really should be a mtn front derailleur if you are using mtn (flat bar) shifters, in my experience.