[Bill Edgeworth]

I have both the $$Proform (basically the same as the LSM) and the cheap Moroso one. The Proform one is nice because you can add or remove shims to adjust the point on the rocker arm where it measures. If you are too far towards the tip it will read low to close to pivot point it will read high.
Having said this I would be more concerned about what fits on your engine and allows you to take readings without having things like master cylinders or shock towers that get in the way of you taking a reading. To me it’s more important to measure pressure on a fresh set of springs/heads and track the changes than worry about the accuracy of a $75 checker opposed to a $300 one.
To me a bench top unit is the only real way to accurately check installed height pressure. But if you are really worried about what the pressure really is when checking on the head you should check your springs at installed height on a bench top unit then install the spring(s) and then calibrate your hand held one by adjusting the shims. (Mike already said this)
Again I have to say I would only use a handheld one for tracking changes in spring pressure not trying to decide if you have 170# or 200#.
This really does not apply to stock but if you are working on a motor that has a different intake and exhaust rocker ratio you will probably get different readings intake to exhaust as the hand held checkers measuring point is in relation to the push rod so you can really only calibrate it to the intake or exhaust rocker on a set up like that. I bought my Proform to check springs on my Mopar because the Moroso one I use on the Pontiac would not read high enough. After spending the money I realized with my fender well headers swooping up I could not check ¾ of the springs without removing the headers. I guess I now have to go out and buy a 90 degree one; it will probably hit the intake!