The biggest difference that you will see with any tester is accuracy.........good ones cost good money.
With the on head , I am looking to see if there are variations in pressure , so it is less critical.
A bench tester has a different job.
And even if it isn't spot on , as long as it is consistent , I could work with it.
But you are getting into test springs , calibration and probably a couple other variables that become important to get accurate and consistent measurements.