Product: again, unless you have sugar, flour or very white objects, I would say that incident metering works very well. You can use incident metering with the flat diffuser (instead of the dome) to check lighting ratios, and when you reach the ratio you need just use the incident metering with the dome for the overall exposure. I suppose for product shot you never have a subject brightness range extending beyond slide film dynamic range as you have lighting under control. If your product is flour, sugar, salt, white flowers etc. then you either close a half stop the incident light metering or you measure with the spot meter and "place" the highlights: I would suggest you place them some 2.33 (if extensive and with important details) or 2.5 stops (if small details) above middle gray.
Portrait: incident metering. Again, if it's a bride with a white dress in full sun light, do close a little to avoid placing the dress at the edge of the film dynamic range. By the same token, any very white object, such as white flowers in a woman hair hit by the main light, which would be placed at the edge of the film detail rendition would require a bit of compensation. Barred this, incident light metering would be by far the best choice as you don't have to worry about the skin tone and how it relates to what the light meters sees as middle tone.
All IMHO and YMMV.