Position control has a lever with a scale with numbers alongside. You can, at least in theory, always get the same 3ph elevation by moving lever to same point along the scale.
The 1/4" inching valve, OTOH, allows you to move the 3ph up or down in 1/4" increments. Again, in theory, you can achieve the same result as position control by remembering how many increments you want to be from a given reference, (such as full up, full down, or where you were before you made the most recent change). Said to be particularly useful for making small adjustments when mounting attachments.
Much debate here as to which is better and the consensus seems to be that it depends upon your intended application.
Hope this helps