The reading is the most important, how it is achieved is also very important, as does the accuracy.
With a sphygmomanometer the operator pumps the cuff up until it is higher than you would expect the patients Systolic pressure to be, then with the stethascope on the brachial artery the cuff is slowly released until there is an audible pulse heard, when you first hear this, it is the systolic or upper BP reading, when you first hear it disappear this then gives the Diastolic reading or the lower number. So the upper reading is the pressure in mm mercury or mm hg that the heart is pumping blood out of the heart onto the artery wall or overcome the resistance applied by the cuff.When the pulse is lost audibly this is the Diastolic or the pressure the system is under at rest.
As you say there is less chance of inaccuracy with an auto bp unit most of the time, but, the reliability you get with simplicity would be my choice if going on an expedition or off the beaten track.