It's been recommended to me by numerous people to check the temp at the glow plug. The fins are there to cool the engine so it makes sense that they would be cooler than the glow plug....they are dissipating heat. The temp of the fins will increase the closer you get to the glow plug.

As Mr. Watkins said, the hottest temp is the only one that really matters because that's where the failure will be if it overheats.

As far as your onboard temp gage goes, what are it's reading in comparison to your gun? Even though the onboard takes reading from the finned head, it may be calibrated to make an adjustment for the difference and display what it calculates the real temp of the engine is.