I don't agree with chemical x that a seven-night cruise would be "awful wrong" at all. Tom makes a good point, however, that cruises of more than seven nights tend to attract a considerably older clientele.
What you do have to remember when comparing one-week cruises with shorter ones is that the latest ships with the most amenities are usually deployed on seven-night cruises, while short cruises are often carried out by smaller, older units. That might be an advantage or a disadvantage, depending on your perspective. For short cruises, IMHO the best ship is Royal Caribbean's "Enchantment of the Seas", based in Port Everglades.