Proteus VSM: digital vs analog resistor and LED
@diegoreymendez is correct. "Analog" simulation is more computationally-intensive than "digital" simulation, to provide for more resolution of what the voltages and currents are actually doing. (It is a fast approximation for "Digital" mode, to speed up simulation.) So set them to Digital if the signals going through them are going to be purely digital, and Analog in all other cases. In this case, the resistor and LED are behaving as analog devices, so for the most accurate results, choose Analog. Here is further reading on Analog-vs-Digital.