I just discussed doing a formal comparison of these with Synergistic Software. They are planning to beef up the Purchase Agent product in the future. These really do represent separate focuses with some overlap. I would say every distributor could use both functions.
In terms of forecasts, Purchase Agent will incorporate MRP Projected Demand data, if it is entered, but this is what it is without any special algorithms like SIA.
SIA does all that cool algorithm stuff to determine what and how much to hold in inventory, and Purchase Agent tells you what and when to purchase with automated processes to cut purchase orders. I would say SIA is for the inventory control function and Purchase Agent is for the purchasing function. Again - separate focus.