The Cross-Hair is a passive tool that reports, constantly, the exact frequency at the cursor position.
The interpolator is an active tool. It always searches the nearest maximum and reports its frequency. It shows the information inside a common help tip, therefore after a latency period from when you stop moving the mouse.
To measure the distance between two peaks with the interpolator, click on the first peak and drag to the other one.
Hold down the Shift key (⇧) and click: the message remains persistent until you click again. Use this trick when you can't read fast enough, but remember that the frequency you are reading doesn't refer to the current position of the mouse.
Press the uppercase J to stamp the message permanently (it becomes an editable note).
To copy the frequency value, the interpolator tool is not required. Control-click and choose Copy Frequency.
To measure a distance and annotate the spectrum with the value, first select a region with the normal pointer, then choose the interpolator tool.
Use the interpolator for a rapid examination of the spectrum. Use the J Manager for a more in-depth analysis.