In a later episode with that Hollow who has those exploding slugs the Zanpakuto can forgive the sins a soul has committed while it was a Hollow allowing them entry into the Soul Society, it can not forgive the sins committed before hand so murderers and the like even when killed with a Zanpakuto are sent to the Underworld. the Quincy only seek to kill Hollows so their weapons destroy rather than absolve the sins.
What happened with Orihime's brother is apparently an "Exorcism" as Rukia implies there is a difference when Ichigo is about to stop Orihime's brother, i think what she said is on the lines of
There's is a difference between killing a Hollow and exorcising one
i've haven't see the entire series or read further in the manga but from the looks of it it's the difference between a Soul Reaper forcing a soul to be purified and forcing the soul to understand what they have done as a hollow over the soul regaining themselves and learning of the sins they have committed, coming to terms with it seeking purification with their own hands, ie. taking responsibility for their own actions