And that from the man, David, who not only has had both, but who 'forced' me to ride his K13S in Spain for a day while he reminisced about the 'joys' of riding an identical K12S to his original.
So, can I give you a non-biased view? No!
I owned a K12S for five years. It had all the deadly issues such as needing new throttle bodies, new airbox and Idle Control Valve as well as flaking paint etc.
I now own a K13S. It has the hot-start issue.
So, I am biased. If you want a non-biased view, you won't find it in an owner's forum. Go to Ash on Bikes and read what the great, sadly late, Kevin Ash said about both of them. He was the single biggest reason that BMW pulled the launch of the K12s back in 2004, recalled all the bikes to Germany (except mine) and did a major rework before the re-launch in 2005. His test ride report says it all, bar the bit about finish as the bikes he rode were all new.
When I spent a couple of hundred miles on Davids' K13S, to me, the difference was chalk and cheese.
Perhaps chalk flavoured cheese because the best bits of the K12 were still there in the K13, but the grin on my face is still there whilst the rather disappointed look on David's face at the end of the day said it all. We both agreed that the killer differences were the wooden chassis and imperfect fuelling on the K12. And this despite my bike being one of the better fuelled K12's. As an aside, Martin's (fjtwelve) K12S was one of the worst I have ridden but is now transformed with a remap.
The relish to go with the cheese was, in the case of the K13, a superb chassis compared to an OK one on the 12, with masses more feel and a much better ability to put the bike exactly where you want it.
When we swapped bikes at the coffee stop, it was as if someone had taken my beloved, but rather careworn bike (which it most definitely wasn't, it was immaculate) and suddenly fixed all the little niggles. New bearings, better tyres, sweeter handling all round. But, of course, most of the bits were the same bits, it's just that the chassis comes alive and the brakes don't have that brick-wall servo effect which I still miss but which detracts from the feel of the stoppers. Things like the shift assist are nice to have and fun to exercise but it is the basic package which is so much better.
Now for the pickle! In my case I was in one and the rest of the trip was spent desperately trying to convince myself that the differences were tiny and the outlay on a newer bike not worth the money or the bother.
Six months later I found my K13S and now every ride is a little bit like that day in the mountains on David's bike.
Brian (who will always hate him for 'making' me ride his K13S but is grateful every day I ride it that he did so
)