As part of our trip from Europe to Oz we rode through India with my GF as pillion in -07 (from Wagah, Pakistan border to the extreme south and then Chennai)... after that we shipped to Thailand, and rode through Thailand (not the north, but I´ve done it several times on another trips).
I can only speak for myself, but for me, out of these two, more or less EVERYTHING has been just, should I say, easier in Thailand! And don´t get me wrong, I love India, too, but it´s a tough country to travel, especially on a bike. Traffic was right there among the very worst on our whole trip. And hey, good luck with your stomach LOL!! To put it short: India (travelling on a bike) seldom felt like a HOLIDAY, but Thailand felt like one most of the time.
Sure if you´d go directly to Leh, and then rent, maybe it´s not the nightmare that would be to get out of Delhi, and then head across the plains to the mountains. The mountains were in fact the only place, where traffic wasnt totally congested, and even that is a bit relative..... but at least bad driving, and challenging road conditions will still remain. Thailand is much more developed in that respect, even though Thai drivers aren´t perfect either.
So for a first-timer for Asia, I´m not sure if I´d recommend India/Himalayas ahead of Chiang Mai. And even if the Himalayas cannot be beat, CM area (for me) is still exceptional, the overall experience I mean. As a European, I´m used to comparing it to the Alps, Pyrenees, or the Norwegian coast, and for me it holds up pretty well. Really just opinions, like everyone else´s, but I think every biker, who loves curvy roads, should go to North Thailand at least once.