Answer:
The science is still not fully clear in this topic, and many new more findings need to be taken into account, but according to most recent studies, guerrilla warfare has led to more PTSD than conventional warfare because of the enemy tactics that are present in guerrilla warfare.
It has been seen that soldiers coming from the Vietnam War and from the Insurgency era of the Iraq War, were more likely to develop PTSD. This has to do with the nature of those wars: they were guerrilla wars in which for example, the enemy could arrive unexpectedly. In convetional warfare, on the other hand, soldiers and generals always have some information about the enemy locaiton.