Schizoaffective normally involves a relatively 'equal' duration of psychotic and affective symptoms and you must have psychotic features without the depression for at least 2 weeks. The delusions/hallucinations may or may not be compatible with mood (mood-congruent).
Psychotic depression, on the other hand, often involves delusions and hallucinations that develop AFTER the onset of depression and are often mood-congruent (e.g. delusions of guilt). Psychotic period must be comparatively shorter than the depressive period.
Differential diagnosis is hard- and there is a great level of fluidity within the diagnoses. I personally view them on a spectrum, with psychotic depression on one end, schizoaffective in the middle and schizophrenia with depressive features on the other end. One can move along the spectrum.
Hope this helps
