I duo a Inquisitor and a guardian, They do take longer to kill mobs but I can duo anything up to orange with out any help. The inquisitor has DPS buffs and good debuffs to help the guardian out. The reactives seem to be pretty good at keeping me up, although sometimes I get hit hard and have to hit my regular heals. I highly reccomend the inquisitor if you want to go that road. They can speed the fight up a bit due to the debuff they have that makes teh mob take damage every time it is hit. (kind of a reverse damage shield) Can't remember the name but it's a post 40 spell.
Disclaimer: I only reccomend this if you have lots of patience, like they stated above the guardian is low DPS. Outlast is the name of the game there.
Best of luck in your decision.