They are not the same. For abilites that have point as target and not an object GetUnitLoc(GetSpellTargetUnit())
will return centre of map.
If you cast an ability on an object you could think they should return the same, but they don't. It would be luck if they would be equal. (I made tests)
Seems like it depends of the coordinates of your cast-click. (you don't have to cover the exact unit-coordinates to make it a target, there is a certain scope as we all know)
So if a unit is a target GetSpellTargetLoc()
and GetUnitLoc(GetSpellTargetUnit())
are similar, but not the same.