In my opinion, I have always felt that Q-torps are more powerful (like 2 or 3 times more then Photons) but since they are "Quantum" torpedoes, they have different damage properties then Photons.
I remember having a discussion with a friend a few years ago about what makes a Q-torp tick. He told me he didn't like the way the ST:DS9 Tech Manual handled how Q-torps worked (can't remember what the tech manual said). He came up with a theory (and an interesting theory at that) about Q-torps that they were actually very tiny "Quantum Singularities" (ie. possible Black Hole) that once they impacted their target, the Quantum Singularity would literly swallow up (ie. destroy) any part of the ships hull into "nothing" until the quantum singularity would burn itself out. This would be ideal against the borg as no matter what the borg did to try to adapt to the weapon, they could never fully adapt to a singularity that would eat up parts of its ship. Now this is assuming Borg don't use sheilds (ie. adaptive Armor or whatever like in SFC3) as he reasoned the Explosive yeild of a Q-torp is only 2 to 3 times more powerful then a Photon against Sheilded targets.
Again, this is just a theory of mine.