Depending on where and why you're cruising, you need to consider the length of the deck. The difference between a long and shortboard is not just a style choice. The deck length impacts your ability to make different types of turns, which ultimately informs the kind of skating you can do.
Video of the Day
Function and Flex in Longboards
The flexibility of the longboard makes it the easier of the two to use for big, broad turns. If your goal is to bomb big hills with carving turns, you absolutely need something 42 inches or longer. The shock absorption the flex in a board this long gives you will also help your knees when you hit rough terrain on downhills at higher speeds.
Turning Agility on Shortboards
For city commuting you'll need something with agility to turn quickly and navigate people and obstacles. Most shortboards (28 inches to 32 inches) have enough flex to get you around the standard town (though you might be walking down certain streets in San Francisco or Paris). The shorter deck length gives you a tighter turning radius which you'll appreciate when a pothole appears out of nowhere.
A lot of new skaters need a little extra stability. A long board is always more stable and easier to maneuver than a short. In her Beginner's Guide to Longboarding, Caroline Nieuwenhuis talks about going from being covered in bruises from shortboarding spills, to commuting with ease on her longboard. She goes on to explain why the balance-challenged or those averse to tricks will like a longboard. She writes it's the solution to the problem of wanting to be a skateboarder but not quite having the skills or desire to sacrifice your body.
The one major advantage of a shortboard over a longboard is how easy it is to transport. A shortboard fits anywhere: in the overhead compartment on an airplane, next to your desk at school or under a table at a restaurant. If you really plan on using your cruiser all the time, this should be a huge factor for you -- no matter how cool your longboard looks, it will always be cumbersome to carry around.