I've got a 2015 SA-T, and came from a 07 1200 GSA12, so hardly comparable.
However, the KTM:
> Has been wholly reliable & fun to ride. The engine is really very good, coupled with decent WP semi active suspension it can basically be ridden as a super sports tourer
> Is well equipped - The S will have a TFT screen which I find well laid out & easy to read, and make sure the quick shifter is activated
> Servicing as mentioned is every 9k miles (big service is @ 18k, 36k etc which includes valve clearances). My minor service cost circa £200 inc labour & vat; Major service cost circa £580 inc labour & vat
> Don't know your insurance position, but mine costs £170 per annum fully comp via Bennetts
> Dealers - I see you're in London, so you have 3 main dealers in reasonable distance (Premier Bikes, Jim Aims & The KTM Centre). I've used Premier & Jim Aims & would recommend them both; never used The KTM Centre so can't comment
I did add a few bits to mine as the T vs the S didn't have LED head lights, quick shifter.
2 items I always add if the bike doesn't have them are a radiator guard & sump bash plate...I bought Evotech versions and they're superb.
Enjoy the test ride...I'm sure you'll enjoy it