Hi Paddy,
I think you will find the main differences between the K13 and KTM ADV are :-
ADV PRO's
* Better riding position for visibility of road - ie your sat higher up and can see further down the road to plan your corners.
I can not emphasise strongly enough how advantageous this is when making "progress" down a nice twisty road!
* Totally unfazed by the condition of the road surface
* Slightly comfier riding position
* Ability to carry more luggage
* Tyres will last a lot longer than the K13 !!
ADV CON's
* A lot more wind buffeting - this may get on your nerves after a while.
* Weaker brakes and front suspension dive ( but you'll get used to this )
* May not be as economical as the K13
You really need to try one as they are not everyone's cup of tea, I've ridden both the K13 and KTM ADV's quite a bit, even had a test ride at Jefferies to compare K13 to the twin cam GS's, link below :-
http://www.ukgser.com/forums/showthread.php?t=240506
Might be worth a read as a lot of the points can translate directly to K13 vs KTM ADV, I have spent quite a bit of time travelling with two guys who own KTM ADV's the bikes are about 4 - 5 years old and seem pretty bulletproof, they have about 18 - 20k miles on them and both their owners don't have a bad word to say about them, also have been very reliable, one guy lives in Germany
and takes his off road quite a bit and the finish looks bulletproof considering the abuse he's given it.
The experience of these two guys was formative in my decision to buy a KTM SMT earlier this year in preference to a GS, as I no longer ride off road I thought this suited my needs better than an KTM ADV, better brakes and road biased suspension and nice sticky tyres!
A test ride will tell you everything you need to know.
Best of luck,
Poucher