Dr R
The 25nm MSA covers the highest point within the splay of the track in that quadrant. The 10nm MSA covers the circle around the airport not on just one quadrant ( unless specified of course ). So, to have a higher 10nm MSA means that if someone is doing an instrument approach, the aircraft might cover (depending on the approach procedure ) the entire 10nm circle over the airport and might have an obstacle or a high point in either one of the quadrants to warrant a higher MSA.
So, if the 25nm MSA is lower, you will only use it if you think you can get a radar vector or clearance within 10nm. Otherwise climb to the higher MSA or stay at the higher MSA till you get further descent clearance or as a part of the instrument procedure.
If the airport does not have a serviceable DME then you will have to stay at the higher MSA.
D6