Unless you have the tiniest, chillest 1.5 year old the world has ever seen, I would buy the kid their own seat. To me, 1.5-2 is *the worst* age for flying, as they're mobile and curious, and still too young to communicate effectively, too young to reason with, too young to stay glued to an iPad for hours, and too young to bribe into good behavior
Or, at least, that was my experience.
My lesson was a 1-hour tarmac delay for a (full) 1-hour flight from DC to NY when my oldest was a couple of weeks away from two. Thought I'd save a few bucks. Biggest mistake of my flying-with-kids career. I can't even imagine 20+ hours of flying to India...