Hello ksw, and welcome to Cruise-Chat; my advise would be to find a travel agent who specializes in cruises and knows the itineraries of the various cruise lines. I am only familiar with Princess and they don't have anything that includes Paris AND Venice. In fact there are only two cruises that offer a Paris excursion: a 14 day London to Rome on Grand Princess and a 15 day Rome to London on Crown Princess. There are many itineraries that start in, end in, or include Venice. If I had to choose, I would definitely take Venice over Paris. Paris is a much better land tour destination because there is so much to see and it is so far from the port. Sailing into or out of Venice is one of the great cruise experiences.
But an agent may be able to find you an itinerary that includes both.
Have fun with your planning . . . it's half the fun!