Key. Like someone else already stated, sometimes I have accidently turned the kill switch off and couldn't figure out what was wrong with the bike later. I try to stay away from the kill switch if I can.
If using the kill switch was part of your routine you would not have been befuddled as to why you couldn't start

If you need to use your kill switch one day you might hit the horn or vis-a-versa, as some here have mentioned.
If you do a professional motorcycle riding course you will be expected to reach all of your controls without looking (eyes closed etc)
Having a distinct routine that engages all the mentioned items diminishes the possibilities for problems:
side stand up and fall over

kill switch off and cannot start

key on, lights left on

petcock left on / reserve, flooding or running out of fuel in the middle of nowhere
I am only making these points in the interest of new riders and returning riders that might come across this thread. The old dogs that already have their tricks/routines don't need this info.
There might be a technical argument about leaving juice in the coils blah blah but I think this is a bit picky on modern motorcycles. I think it is mentioned by Triumph (to use the key) because people forget and leave the key "on", which is probably bad for a number of reasons, not the least of which is not being able to start due to drained battery. Is this any worse than forgetting to put the side stand down and getting your left leg crushed? They probably need a warning/nag light that flashes/beeps when the engine is stopped and stand is up (?)
If we were burning out coils and generally having a problem using the kill switch for these reasons we would have heard about it ad nauseum as surely as the EB kit and ignition switch stuff.
I have never said using the key is bad, I am only going on about using a routine that includes the kill switch to minimize problems. I actually do the key thing last, when I am off the bike, engine killed, side stand down, wheel turned to lock position - then key to lock position and into pocket (I hate looking for the key or leaving it in the ignition).
If any of you blokes fly planes, scuba dive or use guns you will appreciate the value of a proper routine when simple errors or forgetfulness can have very bad consequences.