I have had a GS 650 and my wife has a Transalp.
We toured Europe and can only tell you my experience with these 2 machines.
GS 650 is a better handler imho, but the Transalp is way way better 2 up and for distance. It's like a magic carpet ride. It is less economical than the BM though.
The transalp handling is vague in comparison to the GS, but don't be fooled....if ya smooth then they can hustle.
Transalp is faster flat out and cruising at 80+ is soooooooo easy compared to the GS. It is comfier than the GS and is actually suprisingly good off road (although a Dakar will whoop the Transalp off road, the standard GS is probably more on a par with the Honda).
Personally I prefer the Dakar, simply because I can live with it's "character", but for 2 up I'd take the transalp without a doubt.
I rode back through Spain on the Transalp with my wife on the back and it was actually comfier than my 1150 GS

Put progressive springs in the front of the Tranny with slightly thicker oil, it does improve them quite alot IMHO.