This is a picture of the outlet that you are connected to.
The rounded pin at the top is the ground and the straight pin at the center bottom is the neutral. The two straight slots on each side are L1 & L2 or the two hot leads. Measure from either one of those to the neutral or to the ground and you should read 120V, +/- 10V and if you measure between the two on each side you should read 240V, +/- 20V.
As to what is more accurate, it is probably the Surge Guard, but that is a bit of a guess since neither is likely to be 100% accurate. Most all meters have a tolerance range that is acceptable in terms of design parameters. I am not familiar with the meter you have but most lower price ones are in the range of +/- 5%. But what is key is the Surge Guard since that is what cuts off power. It probably uses pretty much the same voltages as it indicates.
