Just an after thought, however you decide to route the vent, make sure you have a way to clean it out once in a while.
If you run it strieght up through the roof, you might want to have the dryer connect to a clean out "T" so you won't have to disconect the dryer to clean it.
Lint will build up no matter how you run it so, plan ahead so your dryer will run properly.
Jennifer in KC, is right on the money about the moisture.