Where should I discharge my bathroom exhaust to?
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.