I've been planning a gap year to China and India for a while. If you had to pick just one, which would it be? Why?
If you are going to be away for some months you can probably do both - and in fact going to both back to back offers a little world insight since it's hard to avoid comparing them because f their sheer sizes.
I'm not going to suggest which you should visit if you have to make a choice. Pre 15th century these two giants accounted for about 60% of world trade and prosperity, a position they are rapidly returning to. Both have amazing ancient cultures and there is loads to do, see and understand.
But whichever you choose, try to do more than scratch the surface. There are some fantastic classic travel books about both of these places (perhaps a separate question if you'd like some suggestions). about 7 years ago
I agree with Original Nomad, either one of them would be perfect for a long trip, and they are very comparable due to size and population.
I am also not going to tell you which I think you should visit since that is very much your own personal choice, but I would recommend you read up on their cultures and traditions and go to whichever attracts you the most. No matter where you go, eat at the local places and try to meet locals. It will give you a very different insight on their culture, since dealing with the crowds can give you a bad first impression (in over-crowded countries in Asia, people are typically not very considerate in public, but their attitude changes radically when they meet you on a more personal level - you are now someone with a name as opposed to one of thousands that roam the streets daily.) about 7 years ago