We are doing these 2, separately but installing roughly at the same time. Common sense says to me best thing is to go as short as possible, straight up through the roof.
Is there any reason not to go out the side of the house. This would be up into attic, and over, through wall and siding, rather than up to attic and up through roof. I would think drilling a hole in the wall is easier than in the roof and I would think fewer chances of leaks. But I do not want to do this at the expense of ???? I don't know what the consequences are of a longer exhaust. Things nobody wants growing or collecting in there???
Upstairs of bathroom and kitchen is a walk up attic, which one day will maybe become another bedroom or 2, but who knows. This is way down the line. I want to do this logically and assume if we turn this into a bedroom everything is going to have to get re-arranged.
A shorter run is better. Roof caps are relatively easy to install and seal.
Use only rigid ducting with all the joints sealed with foil tape and insulate and vapor barrier them as well.The slip on insulation sleeves work well for this.