Don't know about the other brand engines, but the ZDZ is designed to retain it's highest RPM reading while it's on so you can determine not only ground RPM, but inflight RPM.

The ZDZ unit as far as I know is specific to ZDZ.

As for the small handheld units, I tried a GloBee but found it unreliable with erroneous readings. So I went with the TNC tach now manufacturered by Fromeco....[8D]

http://www.fromeco.org/Products/05FRCTNC/Default.aspx