It depends on your cell. PI is not specific for cell death, it is not a good method to use. Annexin V stainning does not work with all cells, because some cells do not expose phosphatidylserine. You should try some kits for different caspases.
Annexin V staining alone will tell you about early apoptotic population while dual (PI and Annexin V) staining will give the late apoptotic population as well. PI alone in classical sense detects necrotic cell population as during the early stages of apoptosis plasma membrane is still impermeable to PI. The trick would be choose a correct time point. Off course if you want to still differentiate between the two death type, best would be to show caspase 3 activation.
Hi. It is advisable to use both to detect apoptotic vs necrotic cells. If affordable, addition of caspase 3 activity detection gives you a confirmatory assessment. Good luck!