Belief is a choice, while understanding is something I may or may not have, or be able to acquire. I can't simply go to the store and buy it.
Understanding is much more elusive. It seems to appear on its own, as a result of spiritual growth or as a precious gift.
Belief in God is a choice I can control. I like that. I like being able to control something about God. My belief is the ONLY thing I can control about God, so it's good to exercise my choice to believe.
Understanding of God is something I desire. I want it and I work, in my own ways, to achieve it. Listening to other people talk and to my own intuition, a knowledge and understanding of God is building inside me.
But, understanding God is not really the goal, because it's a by-product of prayer and of life; it comes about through experience more than knowledge.