the_wes, you do not say how tall you are or how much you weigh.
Can't speak for the Transalp, cos I have never ridden one but I do ride a Dakar and would have to say that if two-up touring is on the menu and you are reasonably heavy/tall stay away from the BMW's, they are a bit too small. The Dakar has a slightly bigger screen than the GS but it offers very little wind protection for long distance touring. Great around town and an hour or so on the open road but that's about it, IMO. If my missus gets on the back with a few things for a week-end away it's like dragging weed.
I would stick my neck out and say that your estimate of 70/30 split between on/off road, will, in reality end up at an 85/15 ratio, and add a pillion to that and you may just regret buying the single. I weigh 91 kg's and am 1,95m tall, The Dakar is great off road and around town but I like it less and less on the open road and no longer ride two up.