Which organelle is more important, the nucleus or the Mitochondria? Why?

You could make a reasonable argument that either one is more important.   

The nucleus (with its DNA) can be compared to the brain of the cell, since it controls the metabolic functions of the cell and also determines the nature and purposes of the cell. 

The mitochondria are the power supply of the cell.  Although this job sounds a bit more ordinary, the nucleus would be useless without the mitochondria just as the brain would be useless without the digestive and respiratory systems.