Graeme,
I have had my k pouch for 41 yrs so I am one of the 'oldbies'.
I have had my ups and downs, my revisions and complications. I have had problems, especially living here in France. But I have never, every regretted my K pouch.
That said, the j pouch did not exist when I had mine done in 1979. I would not have been a candidate if it had, either.
I had no sphincter at all and had 2 surgeries to try to build me one (Gracillis muscle flap surgeries...Brutal and both failures).
So for me it was either an outside appliance or a K pouch.
I made the right choice.
But I am someone who is very prudent too. If your sphincter is fully functional, if you have strong pelvic muscles or are willing to do the work to maintain them then why would you want to close up the bottom?
A Jpouch is more natural, requires less outside gear and you can eat most anything after about a year (that is the learning curve for the average body).
So, in spite of understanding the desire to have 1 surgery and be on your way with your life, I would strongly suggest that you start with a J pouch and see how that goes. Most people do wonderfully (you mostly hear from those who are having troubles on forums and sites) with their J pouches and do not often have to think about them. Once the learning curve past and the mastery of your new plumbing acquired, you should be quite happy.
Then again, there is always the question of proximity to your surgeon...not a negligible pre-requisite. I have to travel to North America to get help in case of an emergency. And it can be very frightening when you are in trouble.
So, please think carefully, do your research and be at peace with your decision.
Sharon