Everyone in general lean towards roof or gable end of house venting. Then of these two whichever is shortest run..Also everyone agrees down hill slope toward exterior of house..If one is to vent through the roof no house could accomplish this. The slope would inevitably point back to the ceiling. My house has two baths on the front side of house..Currently they vent to the front side of soffit..If I vented them both to the Gable 18ft of pipe would be needed. If vented through the roof 13ft would be needed to get to the back side of the ridge vent..Any advice would be appreciated.
I have a similar issue. The conclusion I have come to is that the shortest available route that is horizontal is probably best if you have to take account of cold climate and that a solid pipe if possible is even better. You will need to note the distance spec of your fan and go for more power if necessary. If you have very cold winters you may have problems with steam condensing in a vertical pipe and dripping back into your bathroom. In any case the vent conduit should be covered with insulation.