If possible and close enough, I would go with the soffit. I worry about snow and ice around the roof vents in the winter. The code says a vent from the drain has to be above the snow line for your area, and they have heat coming out of them, and yet the vents for the bathroom fans are only 5" tall and vent out the bottom. I have used the roof vents without trouble, but I don't like to. Like you mentioned, you have to install the roof vent properly or it will leak. I live in pennsylvania and I worry about the snow. Inspectors around here allow for either, but they watch the length of pipe and they make sure the pipe is sealed all the way to the outside. You should check with your local building inspector, in a lot of areas, you have to get new electrical devices and wiring inspected. Ask him what he likes to see, but I would use the soffit vent if it were my house, if possible. Is the distance to the soffit short enough for your fan and do you have the distance between the top plate and roof to get your pipe in?
mark