I use all the Hornady tools and a couple of others. Have no experience with the Precision Mic.
These are 2 different tools that do 2 different things. One is a headspace tool and the other is an OAL tool.
The OAL tool will tell you your "distance to the lands" if you allow for the headspace of the modified case. For instance when you use the headspace tool on a modified case and a fired case in my 30-06
the measurement has a difference of .010". The fired case has been fired 5 times and I would push the shoulder back .001" when sizing so there is a difference of .009".
So if I took a measurement with the OAL tool I would push the modified case all the way in until it hits the shoulder of the chamber and then push the bullet to the lands. When I retracted it and took the measurement from the case head of the modified case to the tip (or ogive if using a comparator) then the actual measurement would be .009" short. It will depend upon the headspace measurement of your chamber and the particular modified case you get.
If I then expected to seat .015" off the lands by subtracting that .015" from the measurement, I would actually be seating .024" off. Not a big deal since seating deeper does not cause any problems and actually decreases pressure and velocity.
The headspace tool is accurate and will give you a comparative measurement to show how much your case is expanding in your chamber and how much you are pushing the shoulder back during resizing. Very simple to use and fool proof (helps me out!). It does not however measure "headspace" on a belted case but rather the gap between the case shoulder from new case to fired case.
For instance on a 264 win mag I bought as a donor action the gap at the shoulder was significant
if I had kept the caliber and fired this case 3 or 4 times to fully expand it, the shoulder would have moved ~.040"
The "headspace" on this gun as measured to the front of the belt was .007" but that excessive case expansion could have led to premature case head separations and other problems. Useful info
I use the OAL tool to hold the bullet against the lands while I use another tool to measure the distance to the lands. The tool I use is the R-P tool shown here with the Hornady
It is just a stainless steel rod with a brass removable tip. You just insert down to the bolt face (make sure it is cocked and firing pin retracted, don't ask me how I know!) and lock the outside collet
insert the bullet to the lands with the Hornady tool and insert the rod down to the bullet tip and lock the inside collet
measure between the collets
It gives you 2 hard square surfaces to measure between and there can be no mistake (again that helps me out). The only variance you will get will be because of how hard or soft you set the bullet into the lands.