Measuring postsynaptic potential vs action potential at axon hillock?
Measuring voltage at a synapse gives you a measure of the input to a cell.
Measuring voltage at the axon hillock (for a typical neuron, practically the soma can substitute as well) gives you a measure of the output of a cell: how near it is to threshold and/or if it has reached threshold and produced a spike.
Measuring voltage at intermediate sites in the dendritic tree can be a measure of spatial integration, which can be quite complex.
All of these measuring sites can be of interest, though there is a lot of variance in how difficult each will be.