Massive Critical can be quite useful for a wide range of areas. Critical hits are random so it can be unreliable, and some enemies are immune to the instant kill aspect.
Disintegrate is useful because it can give a huge amount of damage on command. It has a long cool down, can be tricky to make sure the charged attack hits the correct enemy. Either you'll go several fights waiting for it to cool down, or you'll run in circles letting until it's ready again.
Short version: Players choice. Massive critical will likely increase your over-all damage output over the course of a dungeon run, but Disintegrate can wipe out weaker enemies in one hit, and take a nice bite out of bosses or tough enemies. If you go at a snail's pace, you can take on single enemies with Disintegrate, but that really, really slows down the pace of a dungeon run.