In my honest opinion, why not both? But I rest my case.I definitely say bad tank/good healer. Aggro can wait most of the time, damage doesn't wait for anyone. But at the same time, if a healer is healing TOO much, then THEY pull the aggro and will probably die very quickly due to low HP carried by healers -- especially in low leveled dungeons.
"Critical fails; for when the GM sobs at night and the players get free checks."