10'' is perfect for a standard 22'' deep vanity top on a standard 21'' vanity. It hits the center of a properly mounted bowl that is not pushed too far back. 7'' would be much too short to reach the middle of a properly placed bowl.

I think it's great to have a longer spout for the sink. Most manufacturers make them so short, you are washing your hands against the back of the bowl. Put in the longer one. I think in the end you will be happier.