PMS colors can be constructed from the four process colors, potentially saving some cost. The decision really depends on your eye for color, as sometimes individual perceptions see a process-mixed color as not as rich as its PMS counterpart. It's best to consult with your printer before making your choice. Decide if you will print web or sheetfed. Web printing uses paper stock that originates from a roll and is finished in either roll or sheet form.
Web printing is fast and good for high-volume work.
Sheet-fed printing uses paper stock that originates in a sheet and is finished in sheet form. Quantity is almost always the deciding variable in this decision, but it is wise to have the job quoted both ways. Check the size of your total package, and any reply devices, for postal conformity as a function of size and weight. Look at the dimensions of your components and work with your printer to develop a press sheet layout. Sometimes changing the size of a component by fractions of an inch can greatly reduce costs.