My favorite is still the old mercury sticks. The stuff (mercury) is toxic but only if you ingest the stuff- keep it in the manometers and all is well. I like them because they work on physics alone, do not need or have any adjustments, and require no checking or calibration. I don't know if you can still get them anywhere but if so, and you feel you can be careful with them, then I would suggest a set of those.
After that, the Morgan type with the metal slugs would seem the best solution. I am sure they work well but would probably calibrate them one time just to be sure all the slugs rise to the same level given the same vacuum (connect all four to one input and they should all read exactly the same). As long as they calibrate one time, I think they would be a great tool just about forever.
Brian
For those who do their own syncs, would like to know which tool you use, ie: dial gauges, bar gauges or electronic types. The make and simplicity of use info would be appreciated.