Simon, I have a 98 1100 and it's been Tourateched to within an inch of it's life, with the addition of Wunderlich lowered pillion pegs, it's pretty much the perfect touring bike as far as I'm concerned.
There are two things that stand out on the Adventure model as I see it.
1. the 1st gear ratio is meant for steeper hills but I can tell you the 1100 can scale some very impressive tracks as it is, but two up it can and will struggle, when running at it's GVM, which I do when two up.
2. The ground clearance is greater and therefore the problem of sliding over rocks when river crossing is less. The same problem when traversing rather wild tracks occurs, but a centre stand bash plate eliminates both problems, to a point that is!
I don't see braided brake lines as a good reason to purchase a new bike, it's cheaper to add them yourself. Also your clutch is operated by a cable and thereby one less thing to leak or rupture.
I personally think if you have an 1100GS pretty much ready for a long trip then it's a better bet than the 1150 or Adventure that you really don't know that well.
The fuel thing is a big plus but it's ramifications as far as fuel range are not known.
The big tank isn't big enough but at least it has a crossover tube.
In essence I think that the 1100GS is really a modern R90/6 in that with a few modifications it is a world class travelling machine, either two up or solo. The Adventure model is the same but it's a version from the factory already on steriods and a bit larger to boot.
I would also offer a suggestion that I haven't seen mentioned on this site before. The addition of a diff breather (reduction axle actually). Our club has a member that manufacturers a breather that covers the breather hole completely, is retained with the use of a 3mm Allan Keyed grub screw, has an "O" ring seal to eliminate leaks either way and enough 5mm outside diameter clear plastic tubing that enables one to run the hose up to the top of the rear suspension breather box area. This hose is held in place by plastic tie downs and the whole shebang takes about 10 minutes to affix. If you're goinbg on a world tour I would recommend this addition as a first modification.
Mick.