Actually, neither. I shot the 12 panels in vertical orientation with the zoom lens set at 135mm equivalent. It's hard to make sure you have good overlap using cloud edges for reference, so I did overkill and shot way more than (probably) was needed. So the pano is 1x12.
My only excuse is my state of mind when I am shooting scenes like this. First, I am an unrepentant sky junkie. Internally, I am raving on like a demented hippie: "Wow. Oh, wow. Look at that!" . . . etc. Second, these are dynamic events: the sun is setting; the clouds are moving (although last night, not too much), so the whole thing is changing and will disappear entirely in just a little while. So I get excited and am simply trying to capture this scene, which I find heart-breaking beautiful.
Finally, to my great good fortune, my better half (aka, the spotter-in-chief) is an enabler. She keeps a weather-eye peeled for these sky events and lets me know if she sees something of interest. After this pano was taken, she was driving us home, and noticed that the sunset had changed yet again. So she looped around to a street with a high elevation in the hopes of getting a better view. Unfortunately, power lines were in the way.
Probably waaaay more than you wanted to know . . . but thanks for asking.